The sex chromosomes: keys to the evolution of human intelligence?

Recent surveys of educational attainment and schooling suggest that girls are outperforming boys all the way through from primary school to A-levels. The reason for this discrepancy, though, remains unknown. Many explanations have been suggested, ranging from the fact that there are simply more boys who fall into the category of disadvantaged" to boys lacking interest, having shorter attention spans, or not being encouraged to take learning seriously by male role-models.

But could there be a simpler explanation? It is no secret that a number of educational and mental disorders have a higher incidence in males than in females, usually because the genes which influence them are linked to the X-chromosome. Imagine a couple, of whom one parent has an X-chromosome gene linked to a particular disorder. If this parent passes on the "defective" gene, this is more likely to affect boys, who have only one X-chromosome (and hence no "good" copies of the gene to counterbalance the defective one) than girls, who will have one defective and one, usually dominant, normal copy of the gene. As a result, the couples' male children may display characteristics associated with the disorder, while female ones typically will not ("normal" genes are no always dominant, but this is often the case).

What does this have to do with human intelligence more generally? Well, if a genes' malfunction is associated with mental disorder, it's function is clearly important to mental functioning. The concentration of these genes on the X-chromosome, moreover, suggests that this chromosome might play an important role in the evolution of human intelligence, and warrants further investigation. A paper by Zechner et al. (2001), published in the journal Trends in Genetics, has done just that, and has found that, in fact, the X-chromosome has a concentration of genes associated with cognition that is over three times higher than that for any other chromosome, even when the authors corrected for possible bias in their data collection.

And a link between the sex chromosomes and intelligence raises some interesting possibilities for its evolution. Zechner et al. (2001) discuss one of these particularly. They note that a link to the X-chromosome may implicate sexual selection in the evolution of intelligence. Sexual selection occurs when individuals of one sex "select" mates on the basis of particular characteristics and/or there is competition between members of the same sex for mates or resources. Both these types of selection (called intersexual and intrasexual selection respectively) can cause the evolution of traits which are unrelated to fitness in the traditional sense; like the male peacock's tail, which is a drain on physical resources, but provides its owner with a reproductive advantage.

Many sexually selected traits, moreover, differ between the sexes. In intersexually selected traits (where one sex chooses mates on the basis of their characteristics), moreover, the possession of a trait in the selected sex must be linked to a preference for it in the other (selecting) sex. Many such traits are linked to the X-chromosome (Zechner et al. 2001). Sexual selection can also occur much faster than natural selection, providing a potential explanation for the speed with which human intelligence seems to have evolved in Homo sapiens - perhaps there is no adaptive explanation for our brains at all, and they are the result of selection for intelligent mates by women...?



References

Zechner, U., Wilder, M., Kehrer-Sawatzki, H., Vogel, W., Fundele, R., & Hameister, H. (2001). A high density of X-linked genes for general cognitive ability: a run-away process shaping human evolution? Trends in Genetics, 17 (12), 697-701 DOI: 10.1016/S0168-9525(01)02446-5

How Shoes Can Change Your Life - And Your Skeleton

A cross-section of a foot inside a shoe. Taken by Mattes, and downloaded from the Wikimedia Commons 26/03/2010.

You might think that shoes can only change your life if you are a sex-and-the-city type shoe lover, spending huge amounts of money on designer footwear. And for most of us, that kind of dedication to shoes is fairly incomprehensible - after all, they're just things to wear to keep your feet safe from broken glass and tarmac, right? Wrong....

In fact, footwear doesn't just change your life in the way that owning that perfect pair of Jimmy Choos can affect a girl. Instead, it can influence the way you walk, the shape of your foot, and even the number and type of pathologies present in your foot bones. A recent study by Zipfel and Berger (2007), for example, has found that some 70% of European males and 66% - that's two in every three! - females has some pathological condition in their big toe, compared to only about 35% of individuals from an archaeological population which habitually walked barefoot.

The study found similar results for all other bones (Zipfer and Berger 2007), suggesting that the habitually unshod foot is healthier than the habitually shod foot in almost all ways. In addition, to ensure that the difference was not due to population differences, they included two other modern (shoe-wearing) populations, Zulu and Sotho, and found similar patterns for all three. The only major anomaly, in fact, was that while all populations (including the unshod one) showed higher proportions of damage to the end of the bones closer to the toes, the Zulu males showed a high proportion of damage to the bone shaft (Zipfer and Berger 2007). The authors concluded that this was because the Zulu population came from a mining town, where males were likely to injure themselves at work.

Perhaps most telling, though, was the fact that high levels of bone deformation or pathological changes that obscured measurement could cause an individual to be excluded from the sample. This is normal in osteological studies: you have to be certain that the measurement you are taking is the same for each individual you study. Interestingly, Zipfer and Berger throw in the fact that while a number of individuals from the three habitually shod populations fell into this category - that is, their foot bones were so damaged they could not be measured - this was true for none of the archaeological, unshod population.

So next time you buy a pair of shoes, take a moment to think if they are really comfortable and properly fitted - it may save you considerable pain later.



References

ZIPFEL, B., & BERGER, L. (2007). Shod versus unshod: The emergence of forefoot pathology in modern humans? The Foot, 17 (4), 205-213 DOI: 10.1016/j.foot.2007.06.002

Linking Footballers, Fingers and Sexual Selection

Footballers, particularly those who play at national or international levels, sometimes seem to have it all: celebrity, fitness, money and success. But rather than just supposing that this is the result of football's cultural status and importance, researchers have also suggested that it is the result of natural selection - not the survival of the fittest, as modern medicine and cultural systems ensure that in the Western world at least, most people have the chance to live, but perhaps the success of the fittest.

The argument runs like this. In prehistoric times, humans were subject to both natural and sexual selection. Sexual selection works in two ways. Firstly, there is mate choice (intersexual selection), which may be expressed by one or both sexes - i.e. either men or women or both may select their mates according to certain criteria of judgement. Secondly, within a sex (usually males) there may be competition for resources, particularly those which enable access to mates or successful child-rearing. This second form of selection, called intrasexual selection, is what produces fighting between the males of many species. Many Western societies now frown upon direct competition in terms of fighting, but the characteristics which make for good fighters likely remain - and may be expressed as prowess in sports which require high levels of spatial judgement, speed, endurance and strength. Football may be one such sport.

If this was the case, we might expect to see correlations between the levels of the male hormone testosterone, often associated with strength and other typically male characteristics, and football ability. Testosterone has also been linked to the formation of an efficient cardiovascular system in men, making it potentially even more important for fighting and/or football playing, although its action upon these systems occurs before birth (pre-natally), and therefore cannot easily be measured for large samples of football players.

To test for a link between football (or sporting) abilities and testosterone levels, then, researchers have to be a little more creative. The paper I have recently read on the subject, published in 2001 by Manning and Taylor, for instance, looks for correlation between sporting abilities in football players of various standards and the ratio of the second digit (the index finger) and the fourth digit (the ring finger). This ratio, written as 2D:4D, is typically lower in men than in women. That is, men tend to have shorter index fingers relative to the length of the ring finger while women have the opposite. This ratio has been explicitly linked to testosterone levels during foetal development, and does not change after a child is born (barring accidents involving the fingers), making it a good proxy for prenatal testosterone levels (Manning and Taylor 2001).

Manning and Taylor, therefore, carried out three studies looking for a link between the 2D:4D ratio, sporting ability (particularly in football) and visual-spatial judgement, thought to be an indicator of high "fitness" in men. The first two of these studies used participants from sports centres and libraries, and asked them to rank their sporting abilities on a scale from 10 ("I have represented my country") down to 0 ("I do no sport"). The first of these studies found a link between 2D:4D ratios and sporting scores such that the higher the score a participant gave the lower the digit ratio was, and hence the higher their pre-natal testosterone exposure likely was. The second found a similar link between visual-spatial judgement scores and 2D:4D ratios. Both these relationships were quite variable (with participants at a particular sporting level having varying digit ratios), but were statistically significant, that is, highly unlikely to have arisen due to chance alone.

The third study, meanwhile, examined footballers specifically and ranked them according to their league and coach references. It also involved a "control" group of non-footballers, for comparative purposes. It found that there was not only a difference in 2D:4D ratio between footballers and controls (footballers had lower average digit ratios), there was also a decrease in digit ratio the higher up the sporting scale the footballer was. So international players had lower 2D:4D ratios than players in the premier league, who had lower ratios than first division club players, and so on. Coaches, interestingly, fell between internationals and premier club players, suggesting (as is indeed the case) that they would have been highly successful footballers themselves (Manning and Taylor 2001).

So, all this suggests that footballers, and sporting professionals in general, are successful because they are some of "the fittest" in an evolutionary sense; they have high pre-natal testosterone levels, and hence well developed "fighting" skills which can be transferred into sports. Manning and Taylor also note that there are two possible explanations of this high fitness. Firstly, as study two suggested, there may be a link between visual-spatial awareness and pre-natal testosterone levels. Alternatively, or as well, the role of testosterone in the development of the cardiovascular system may be important. Both hypotheses are supported by some evidence (for example, that exposure of male foetuses to female hormones in the womb can lead to both digit anomalies and malfunctions of the cardio-vascular system), but we cannot yet discriminate between them. Still, if the great social institution and economic phenomenon that is international football could have arisen as the result of selection for male fighting abilities, we may be looking too hard for direct evolutionary explanations of other modern human traits like culture and language. Perhaps they, too, as some researchers suggest, were in part the by-products of selection for other features.



References

Manning JT, & Taylor RP (2001). Second to fourth digit ratio and male ability in sport: implications for sexual selection in humans. Evolution and human behavior : official journal of the Human Behavior and Evolution Society, 22 (1), 61-69 PMID: 11182575

It’s Official – Fathers ARE Important to their Childrens’ Upbringing

David Cameron’s “Broken Britain”, with its image of moral decay driven by the breakdown in family life and poverty, may be inciting a lot of debate in parliament and the public press, but to read many studies of human evolution, you might be mistaken for thinking that the human male has never actually played a meaningful role in childcare. Most evolutionary studies focus on female life history – age at first reproduction, number of offspring and interbirth interval, for example – to the exclusion of the fathers. Those studies which do consider the role of male care in the evolution of human populations usually suggest that their role is indirect, that is that they provide food or other resources to their wife and kids, but are not involved in child rearing directly.

A recent paper in the American Anthropologist focusing on the potential importance of direct parenting by men (Gettler 2010), is therefore a refreshing novelty. It notes that modern humans are unusual among mammals in having both a long childhood (requiring more input from caregivers) and a relatively short interbirth interval. This suggests that individuals other than a child’s mother are likely involved in their upbringing, thus reducing the pressure on mothers and enabling them to have more children.

In many modern human societies, these additional caregivers are fathers (as well as other relatives), but evolutionary hypotheses largely assume that female behaviour is the most important factor in changing reproductive behaviour. The “grandmother hypothesis”, for instance, proposes that the extended post-reproductive lifespan of women caused life-history change, by ensuring mothers could rely upon their own female relatives. Another such model, the “allomother” hypothesis, suggests that other females – maybe young ones practicing their childcare, or other members of the group – were the key. Care by both parents has also been suggested as an important factor, but only relatively recently (Gettler 2010).

Gettler’s hypothesis, though, is slightly different – he suggests that it was men, not women, who caused the change, by getting involved, perhaps for the first time, in the direct care of children, in particular by helping females to carry offspring during population movements. His research is based on assessment of energy expenditure in Homo erectus and later species in our genus (working on the assumption that earlier species likely reproduced with a longer interbirth interval similar to that of a chimpanzee), and builds on the idea that for large bodied hominins, the key tactic to reduce the energetic cost of each offspring was to “stack” them, reducing interbirth intervals, weaning infants earlier and thus lactating for shorter periods. Overall energetic costs of living were relatively high in Homo erectus, especially compared to earlier species with smaller, less “expensive” brains, but reductions in gut size suggest that both trade-off between organs and increased dietary quality were acting to counterbalance the increased cost. This has led to the traditional model for life-history change, which proposes that this increased quality diet relied upon meat, which was hunted by males and given to females by the hunters. This would lead to monogamous pair-bonding, division of labour, and, thereby, to shorter interbirth intervals as women and children were no longer subject to the same selective pressures as their earlier counterparts. More recent studies of hunter-gatherers, in contrast, suggest that gathering provides more calories in the day-to-day life of groups, and researchers now are uncertain whether pre-modern humans would have been sufficiently efficient hunters for this provisioning model to be correct.

Gettler’s model, though, recognises that in fact carrying infants – especially in hunter-gatherer societies where travel distances per day can be long – may be more energetically expensive than lactating, and is not usually alleviated by division of labour. Instead, he proposes, when groups moved around, it was the men who carried the offspring, reducing female energetic costs dramatically and thus (indirectly) enabling them to bear more children with shorter gaps between births. Those males who thus became directly involved with their childrens’ upbringing would have a fitness advantage over those who did not, producing more offspring and perpetuating the behaviour, particularly where males and females were foraging together and ranging over large areas.

In addition to this new model, moreover, Gettler (2010) also notes that this model emphasises the potential complexity of male-child relationships. The energetic benefit to the mother of male carrying of children only pertains in certain circumstances, for example, where foraging is roughly equally efficient in both sexes and hunting is not male-dominated and frequent. This is, in my view, even more interesting than the suggestion that direct male care was important, as it suggests that life-history models are finally coming into line with other fields of palaeoanthropology, in which the complexity of evolutionary processes have been the subject of increasing certainty in recent years. Behavioural and cultural flexibility, and the occupation of variable environments have been emphasised in models of human physical evolution for a few decades now, but life history research has remained focused on the savannah hypothesis until very recently.

I’m not sure what the implications are for the Tories’ social policies on Broken Britain, though....



References

Gettler, L.T. (2010). Direct male care and hominin evolution: why male-child interaction is more than just a nice social idea. American Anthropologist, 112 (1), 7-21 : 10.1111/j.1548-1433.2009.01193.x

Human and Chimpanzee Handedness

Of the many mysteries surrounding human evolution, the question of why humans, alone out of all the apes, display a strong tendency towards being right-handed is perhaps less well known than uncertainties about our locomotion, brain size and cultural capacity. Yet the fact remains, over 90% of humans are right handed, and strongly so - there are proportionally few left-handed individuals and very few ambidextrous ones. Handedness is a manifestation of laterality - having a behaviourally dominant side or limb - and may be related to the relative dominance of the two halves of the brain. In humans, who are (mostly) right-handed, the left side of the brain, which is the side associated with language, is therefore dominant.

In chimpanzees and other apes, though, the situation is different. Laterality is reduced (with many individuals being ambidextrous), and although the results of early research are inconclusive, there is no demonstrable preference for being right-handed over being left-handed (Braccini et al. 2010). In fact, these authors, publishing in the current issue of the Journal of Human Evolution, argue that much more research into handedness in the apes is needed to establish whether the human predominance of the right side of the body is an extension of a trait present in the last common ancestor or a uniquely human character.

Braccini et al. therefore set up an experiment in which a number of chimps (of whom 15 were ambidextrous, 15 right-handed and 16 left-handed according to previous research) were given sticks to access peanut butter in the middle of plastic tubes. Each chimp was tested in three different postures: for the first, they were allowed to hold the tube and all sat down to extract the food; for the second, the tube was suspended vertically above head-height but within a short distance of a wall, so they could support themselves with one hand while standing bipedally, and finally, the tube was suspended above head-height but away from the wall, so the chimps had to stand unsupported (Braccini et al. 2010). The research found that the degree of laterality (preference for one hand over the other) increased significantly as the chimps moved from seated to supported and unsupported bipedalism and from supported to unsupported postures, but the level of right-handedness in the group, interestingly, did not - in fact, although there was a slight increase in the proportion using their right hands to access food when standing bipedally, on the whole, the change of posture merely strengthened the chimps' earlier hand preferences.

The most interesting implication of this study, of course, is that while it does not disprove the widely-held hypothesis that tool use has driven human lateralization, it does require that an additional factor be invoked to explain the high proportion of right-handed people. In fact, Braccini et al.'s paper suggests that a change to bipedal locomotion, especially if associated with tool use and manipulation of the environment, might indeed have enhanced lateralization in early hominins substantially. The subsequent change which led to 90% being right-handed might, in fact, be the enhanced lateralization of the brain which accompanied the origins of language - a skill primarily located in the left half of the brain. While Braccini et al.'s paper does not provide any direct evidence for this change, it does support the existence of a second, directional, shift in lateralization which, furthermore, must have arisen after the human-chimpanzee split. However, this problem is of a chicken-and-egg nature - we cannot know whether cerebral lateralization occurred before, or was enabled as a result of, increases in right-handedness and left-brain dominance. In addition, it may be that there is no selective advantage to being right handed - because this is the result of selection for other features. It will be interesting to see how the debate turns out!



References

Braccini S, Lambeth S, Schapiro S, & Fitch WT (2010). Bipedal tool use strengthens chimpanzee hand preferences. Journal of human evolution, 58 (3), 234-241 PMID: 20089294

Fossilisation and Vegetation Patterns: Another Study of Decay and its Implications

Following on from my recent post about the decay of chordate animals, I have encountered a related paper, this time from Quaternary Research and focusing on the preservation of plants in middens (rubbish dumps) constructed by woodrats. This paper, written by Nowak et al. (2000), explores the question of how well these middens represent the vegetation surrounding them, by developing a method which calculates the probability that species that are missing from the midden are actually not present in the landscape.

To do this, Nowak et al. carry out two surveys. In one, they examine 27 woodrat middens less than 200 years old and compare them with the vegetation that currently surrounds them, and in the other, they compare middens from the same location and of the same age. From the first study, they obtain a "best case" scenario, which allows them to estimate the upper limit of the probability that midden remains accurately represent the surrounding vegetation and the lower limit of the probability that the midden is not representative of vegetation (Nowak et al. 2000). From the second experiment, which is more realistic (because the authors do not actually know the exact nature of the vegetation surrounding the midden), they can calculate the lower bound of the probability of accurate representation and the upper bound of the probability of inaccuracy - a worst case scenario.

Their findings are promising for palaeontologists interested in plant fossils. Overall, the probability of a false interpretation of vegetation pattern based on midden composition is between 7 and 11%, and for some species, it is between 0 and 6%. The grasses are the obvious exception to this, however, with inaccurate representation of grass species potentially as high as 40% (Nowak et al. 2000), although for a minority of species the results were inconclusive because of difficulties identifying fossil specimens, random fluctuations in probabilities, small sample sizes or (possibly) selection against those species by the woodrats. At the same time as these results are promising for palaeontological investigations of vegetation patterns, and potentially extensible to other organisms (like animals), however, I cannot help but feel that for most species - particularly those which are now extinct - carrying out analyses to this level of detail will be impossible and making assumptions based on studies of extant relatives potentially risky; there is no way to know how much we can generalise from results like these. That said, any moves forward in the study of palaeontological data quality are highly valuable, and, if these studies are continued, they may prove useful across the board at least in providing a ballpark probability that absence from a site actually implies absence from the surrounding area. I will be watching out for further studies of this type focusing on mammals - woodrats do not range far from their middens, so the scientists only had to evaluate 100m circles of vegetation, but I imagine surveying the surroundings of recent mammal assemblages will be much more arduous.



References

Nowak, R. (2000). Probability That a Fossil Absent from a Sample Is Also Absent from the Paleolandscape Quaternary Research, 54 (1), 144-154 DOI: 10.1006/qres.2000.2143

Decay Processes and Chordate Phylogeny

I have just read a Nature paper reporting some experimental work studying the pattern of decay in two soft-bodied species, Lampetra and Branchiostoma, which are thought to be the best proxies of the early chordates (chordates are the group of animals that includes the vertebrates and those invertebrates that are their closest relatives).

The authors, Sansom et al. (2010), note that our understanding of the early evolution of the chordates is very sparse, in large part because the early chordates were entirely soft-bodied and are only rarely preserved, and in part because the interpretation of those soft-bodied fossils we do have is complex. It is especially hard to distinguish the earliest true chordates from their close, non-chordate relatives (called the "stem chordates"). They suggest that this might be rectified by better understanding of the sequence in which features of early chordates decay. In particular, we need to know whether the characteristics which characterise the true chordates decay relatively fast upon the death of their bearer, as if this is true, the partially-decayed true chordates will be misinterpreted as stem chordates, which they now resemble (Sansom et al. 2010).

And, indeed, this is exactly what Sansom et al. found was the case. In their experiments, which tracked the order in which features of the two species decayed, those which were lost first were those which were most informative about the relationships between early chordates. As a result, the relative abundance of stem chordates in comparison with true chordates in the fossil record may be the result of the incomplete preservation of the crucial characteristics which would enable researchers to identify their real relationships.

I think this paper is fascinating. At the same time, though, if it is true that the characteristics which are most informative about early chordate evolution are those which decay first, it is difficult to see how we will every sort the true chordates from their stem chordate relatives, barring finds of even more exceptionally preserved fossils than we already have from the relevant period. Despite this, knowing more about taphonomy (the processes of decay and destruction that affect dead organisms) can only inform our reconstructions of the evolutionary history of life, even if some parts of that history can never be fully resolved.



References

Sansom, R., Gabbott, S., & Purnell, M. (2010). Non-random decay of chordate characters causes bias in fossil interpretation Nature, 463 (7282), 797-800 DOI: 10.1038/nature08745

The Anatomists

This weekend, I watched the first episode of The Anatomists, a Channel 4 three-part series on the history of anatomical investigation. Having studied human anatomy and osteology during my masters' degree and since I use some anatomical work in my PhD research, I thought it would be really interesting to find out a little more about the history of the discipline, and I wasn't disappointed. It was definately worth a watch. The program, although only about 50 minutes long, covered the history of anatomy from its grisly beginnings in human vivisection and the treatment of gladiators' wounds through the Renaissance and the beginnings of University study of the human body and all the way up to the eighteenth century (where the second program will take over).

At the same time as The Anatomists was interesting and clearly well researched, though, I did feel the programme went a little overboard on the dramatic front. The presenter toured an anatomy theatre by the light of a small electric torch while extolling how unpleasant it would have been to attend an anatomy; the program was accompanied by montages of anatomical images which didn't always contribute to the dicsussion at hand, and there were several references which, seemingly unconcernedly, described anatomists as "obsessive" or "perverted" showmen. Of course, early anatomists must have had strong stomachs, and indeed a flair for working in public (as dissection has always been right in the public spotlight), but documentaries have to walk a fine line between truth and shock value. It was a pity, in my opinion, that The Anatomists, which overall was both interesting and convincingly serious, occasionally fell into the trap of sensationalism, especially since the subject already has a somewhat shady reputation among the general public. I think a strong relationship between science and society is vital for both parties, and feel that a non-sensationalist, balanced representation of research - particularly where ethical questions are involved - is particularly important to that relationship.

The Emergence of Human Limb Proportions

The current issue of PNAS carries an interesting paper on the evolution of human limb proportions. The authors, Young et al. (2010), propose that one key change in the evolution of humanlike limb adaptations is a reduction in the strength of the developmental links between fore- and hindlimbs, and moreover, that this change actually occurred in a non-hominin ancestor we shared with other great apes.

The quadrupedal primates, like most vertebrates, have strong serial homologies between their limbs. Each limb is composed of three units, specifically the thigh/arm, the leg/forearm and the foot/hand, which are, in most species, tightly coupled such that changes in the relative proportions of the parts of the hindlimb will bring about corresponding changes in the forelimb and vice versa (Young et al. 2010). Humans, in contrast, have differently proprtioned fore- and hindlimbs, with the patterns linked to their different functions in fine manipulation and bipedal locomotion respectively. In addition, the fossil record of human evolution suggests that the changes from the ancestral pattern occurred in an evolutionary mosaic, with fore- and hindlimbs changing independently and at different times, in response to separate selective pressures (Young et al. 2010).

For me, the most interesting part of this article is not the proposal that the move towards weaker coupling of fore- and hindlimbs was important to human evolution (as this seems fairly straightforward, although interesting), but Young et al.'s suggestion that the change actually happened in a human-ape common ancestor rather than within the hominin clade. Many great apes do have functionally differentiated and differently proportioned fore- and hindlimbs, likely as the result of a reduction in the number of pleiotropic genes. Pleiotropic genes are those which influence more than one anatomical structure. This, to me, suggests we are justified in spending more time developing our understanding of the locomotor anatomy of the great apes.



References

YOUNG, N., WAGNER, G., & HALLGRIMSSON, B. (2010). Development and the evolvability of human limbs Proceedings of the National Academy of Sciences, 107 (8), 3400-3405 DOI: 10.1073/pnas.0911856107

Ecological Niche Modelling - Friend or Foe?

Problems associated with low spatial and temporal resolution in datasets are a daily hazard of my particular field of research, palaeoanthropology. The fossil record, as everyone knows, is hugely incomplete and, in addition, biased. Those records we do have about the biogeography of extinct species, in particular, are usually patchy and likely to be biased in favour of those parts of the distribution where fossilisation was probable and disturbance since sufficient to uncover the remains but not so marked as to destroy them. It's a tall order - rather like Goldilock's requirements of porridge - and it's hardly surprising that only a small proportion of organisms become fossils that are found by researchers today.

Those who work with the past, however, do tend to ignore the very similar problems facing biogeographers whose target organisms are still extant. This morning, though, I came across a paper reporting a piece of research into the use of museum collections to fill gaps in scientific knowledge of biogeography and hence to improve both conservation efforts and ecological understanding.

The research, by Newbold (2010), focuses on one particular technique, called ecological niche modelling, which I have always assumed would be particularly useful to palaeontologists. Essentially, ecological niche modelling was developed to "fill the gaps" in our knowledge of a species' distribution. If we plot every known occurrence of a particular species, for example, the resulting distribution will be incomplete, because we are unlikely to have sampled every possible site where that species might occur. Some of the apparently empty sites on our distribution map, then, will actually represent sites that are not sampled. There are a number of ways we can deal with this. Most simply, we can ignore unsampled sites by assuming they are empty, although this is unreliable (Newbold 2010). Instead, then, models can be developed to assign each unsampled site a value (presence/absence or occupied/empty). This assignment can be random, or it can employ an ecological niche model, which analyses the distribution of known presences in light of their environmental conditions to identify a set of rules that describe an organisms' distribution in terms of its context. So, for example, an ecological niche model might determine that all occurences of species X are in woodland and within 20km of a water body, and then can use these rules to decide which unsampled cells are likely to be occupied.

So far, so good. For palaeontologists, this technique holds potential - it would allow us to patch some of the gaps in species' distributions that are the inevitable result of using fossil data. However, Newbold then goes on to discuss the limitations of museum data in ecological niche modelling, which I had not yet thought much about. Museum collections are exactly what palaeoanthropologists would be working with: our fossils are kept in collections, with location data and environmental reconstructions published in the associated literature. However, as Newbold quite rightly notes, the records kept by curators and museums, particularly where the fossils were discovered a long time ago, may be both biased and even incorrect. For example, those fossils which were found are those which eroded from rock faces, but only where there were people to find them. Certain areas of Africa, for example, are likely to be poorly sampled by palaeoanthropologists because they are politically unsettled or hostile to Western nations, so any fossils that have emerged are unlikely to have been recognised. This is only a problem where the bias favours certain palaeoenvironments and hence affects the rules produced by the model(Newbold 2010), but, as of yet, we cannot know whether this is the case in palaeoanthropology.

In addition, small errors in the location records of fossil finds may also affect our models (Newbold 2010). The Taung child, the famous first fossil of Australopithecus africanus, for example, was famously found in a limestone quarry in South Africa by workers - it's exact location was never noted. In addtion, prior to the use of GPS, many fossil findspots were difficult to locate with the accuracy possible using modern technology. This georeferencing problem is particularly common (Newbold 2010).

Now, I would argue that these problems in their own right do not invalidate ecological niche models, particularly where - as in palaeoanthropology - there are limited opportunities for obtaining better sampling of distributions. But just after finishing this paper, I encountered a second, this time in the Journal of Biogeography, which highlights the dangers of niche modelling in a very different way. The authors, Lozier et al. (2009) have constructed an ecological niche model based on sightings of the sasquatch - bigfoot - to explore whether reasonable distribution models can be constructed from questionable observational data. The distribution their model produced, in fact, was very successful in tests (proving to be capable of producing a set of ecological rules which matched the conditons of all but one of over 500 sightings), and was very similar to that of black bear, despite being based only on uncertain sightings of a creature that has never been proven to exist. The paper, overall, not only gives grounds for serious thought about the use of uncertain data in ecological niche modelling, but also enables its authors to propose that bigfoot may, in fact, be a misidentified black bear....

Clearly, it is very important to critically assess the nature and quality of data used in ecological niche modelling if the technique is to be useful and produce reliable results. This may particularly be the case in palaeoanthropology, where taphonomic processes (which are inherently biased towards certain palaeoenvironments) have been involved, even where we can be pretty sure that the subjects of the model actually existed!



Reference

NEWBOLD, T. (2010). Applications and limitations of museum data for conservation and ecology, with particular attention to species distribution models Progress in Physical Geography, 34 (1), 3-22 DOI: 10.1177/0309133309355630

LOZIER, J., ANIELLO, P., & HICKERSON, M. (2009). Predicting the distribution of Sasquatch in western North America: anything goes with ecological niche modelling Journal of Biogeography, 36 (9), 1623-1627 DOI: 10.1111/j.1365-2699.2009.02152.x

The Biological Species Concept and Hybridisation in Primates

The most popular species concept in use today, the Biological Species Concept (BSC) defines a species through reference to the limits of reproductive compatibility: essentially, through the idea that any pair (male and female) within a single species will be capable of producing viable and fertile offspring, while a couple which belong to different species will not. The boundaries of successful reproduction, then, can be used to delineate species, at least in sexually reproducing animals.

Of course, it's not actually that simple. Baboons, for example, of the genus Papio, have been the subject of extensive debate, with some authors recognising as many as five separate species on the basis of morphology while adherents of the BSC, while not denying that these putative species are constant and stable, note that hybridisation between the various populations Papio means that only on species can be present. So biologists who favour the BSC lump all members of the genus Papio into one species, despite their differences, while many other researchers do not (Jolly 2001).

Interestingly, though, Papio beboons do not only hybridise with one another. Dunbar and Dunbar, for instance, noted as early as 1974 that apparently fertile and reproductively successful hybrids can be produced between at least one Papio species and the gelada baboon, in the genus Theropithecus. These two genera are closely related, to be sure, next to one another on most phylogenetic trees of the old world monkeys, but have been distinct lineages for several million years. In addition to Dunbar and Dunbar (1974)'s wild hybrids between the gelada and anubis baboons moreover, Jolly et al. (1997) report hybrids between hamadryas baboons and geladas in the wild, and Markarjan et al. (1974) between Papio baboons and both geladas and rhesus macaques, the baboons' even more distant relatives in the genus Macaca. These so-called "rheboons", however, may not be fertile or capable of attracting mates (Jolly 2001).

In light of these papers, I have been reading about hybridisation in monkeys, and it seems to be a lot more prevalent than I previously realised (there are papers galore out there, but to go into detail on all of them would take far more space than I have here!) At the same time, though, I started thinking about this after reading Jolly's paper from 2001, as cited in the last post, which is about the use of papionin monkeys as analogues for our ancestors. One interesting suggestion of Jolly's is that if baboons and macaques, or baboons and geladas, can hybridise after several million years as distinct lineages, why do we believe that all the species of australopithecine-type hominins (genus Australopithecus and genus Paranthropus) necessarily behaved as biological species? Or, for that matter, why do we think that early species of our own genus, Homo, couldn't have hybridised with one another? To me, although there is no clear evidence for hybridisation in our own lineage, there is no reason to rule it out; the evidence simply isn't clear enough. However, it is difficult to envision what evidence we might find that would inform us about hybridisation in past hominins. I suppose the real question is whether the morphological differences between the hominin species we have already identified are real discontinuities or simply artifacts of the incomplete fossil record.

Whether we can tell from the fossil record or not, this is interesting stuff, and has substantial implications for hominin taxonomy and our understanding of the evolutionary process.



References

DUNBAR, R., & DUNBAR, P. (1974). On hybridization between Theropithecus gelada and Papio anubis in the wild☆ Journal of Human Evolution, 3 (3), 187-192 DOI: 10.1016/0047-2484(74)90176-6

JOLLY, C.J., WOOLLEY-BARKER, T., BEYENE, S., DISOTELL, T.R., & PHILLIPS-CONROY, J.E. (1997). Intergeneric hybrid baboons. International Journal of Primatology, 18 (4), 597-627

JOLLY, C. (2001). A proper study for mankind: Analogies from the Papionin monkeys and their implications for human evolution American Journal of Physical Anthropology, 116 (S33), 177-204 DOI: 10.1002/ajpa.10021

MARKARJAN, D., ISAKOV, E., & KONDAKOV, G. (1974). Intergeneric hybrids of the lower (42-chromosome) monkey species of the Sukhumi monkey colony Journal of Human Evolution, 3 (3), 247-255 DOI: 10.1016/0047-2484(74)90183-3

Do extant apes make good models for our ancestors?

There seems to be some debate in the recent literature (since the publication of Ardipithecus ramidus) about the use of non-human primates as models for hominin ancestors. The argument seems to arise from the traditional assumption of palaeoanthropologists that chimpanzees have remained "more conservative" in anatomy, behaviour and ecology than have modern humans, making them informative about the last common ancestor of the two lineages. The discoverers of Ardipithecus, in contrast, suggest that the distinctive anatomy and paleobiology of that taxon suggest that the last common ancestor (which must have lived only a short time before) could not have been troglodytian; indeed, it likely had a different diet, a novel locomotor pattern and a habitat preference very different to that of modern chimpanzees (Hanson 2009).

In the January edition of Science, there is a reply to this assertion by the Ardipithecus authors, in which several primatologists and palaeoanthropologists assert that, in fact, the end of comparative studies in the field is not nigh, and that much remains to be learned from the study of extant hominoids (Whiten et al. 2010). Some of the authors of the Ardipithecus papers then reply, to say that of course they didn't mean there was no merit in studies of extant taxa, and, in fact, strongly support that work - but not its use as the source of direct models for hominin ancestros, which must instead be studied from the perspective of "fundamental evolutionary theory" (Lovejoy et al. 2010).

I must admit to being a little confused by this exchange. I read the Ardipithecus papers, and found no suggestion that all studies of extant hominoids were necessarily redundant; similarly, I would have thought that evolutionary theory ought to have played a substantial part in palaeoanthropology even before the direct comparison of chimpanzee and last common ancestor was disputed. Jolly (2001), I think, makes a good point that valuable analogies about the evolution of key hominin features can be drawn from a wide range of primates and mammals (not limited to, but not excluding the extant hominoids). what is needed, it seems, is more awareness of how those analogies are constructed and used - researchers using analogy are not saying "we are going to assume that this extant taxon is representative of a particular ancestral taxon". That would not get past the peer review system of most journals, I suspect, as we know it is not typical of evolutionary theory. Chimpanzees are not our ancestors, but they may be more similar to them (or indeed, more different) than we are. At the same time, analogies, of the form "in a certain characteristic, X is to Y and A is to B" can be dran from any taxon - Jolly suggests Theropithecus baboons - and do not make any assumptions about either the last common ancestor (which may be extremely distant), or the rest of the characteristics of the two organisms that are being compared.

In summary, modern apes clearly can shed some light on our evolutionary history, as they are phylogenetically our closest relatives (a point to Whiten et al.) But they are not necessarily analogues of our last common ancestor in any respect, and certainly are unlikely to be so in all respects (as Lovejoy et al. and Hanson have noted). The term "analogue" implies a certain type of relationship between compared taxa, but the term "model", at least to me, does not; it is more general, and might refer either to analogue studies or to more general comparative studies. Perhaps the palaeoanthropological community simply needs to be more careful about its use of comparisons and terminology?

References

Hanson, B. 2009. Light on the Origin of Man. Science volume 326, pages 60-63.

Whiten et al. 2010. Studying Extant Species to Model our Past. Science volume 327, page 410.

Lovejoy et al. 2010. Response to Whiten et al. Science volume 327, pages 410-411.

Jolly, C.J. 2001. A Proper Study for Mankind: Analogies from the Papionin Monkeys and Their Implications for Human Evolution. Yearbook of Physical Anthropology volume 44, pages 177-204.

Redefining the Pleistocene

I've think I heard the rumours a while ago, but it's only the last couple of days whil writing about australopithecine landscapes that I've actually clicked - the re-arrangement of the Plio-Pleistocene boundary last summer (from 1.8 million years ago to 2.6 million years ago) has ensured that the australopithecines are now almost entirely Pleistocene species.

The genus Paranthropus, in particular, barely appears before the Pleistocene boundary under the new rules. Suddenly, almost every time I've used the term "Late Pliocene" in my literature review needs to be revised to "Early Pleistocene"....

Apart from this little irritation, though, I'm not sure what other impacts the change will have - except in making textbooks out of date, which, frankly, happens fairly often in palaeoanthropology as a result of fossil finds anyway. At the same time, I can completely understand that palaeoanthropologists, and palaeontologists for that matter, are angry about not being consulted on this change - it's impact will not only be felt by the geologists who define blocks of geological time, and the decision probably shouldn't have been taken by one group of stakeholders in isolation from others.

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The statement by the International Union of Geological Sciences re-defining the Pleistocene can be found here:

http://bit.ly/cqWHgn

How Important is Temperature?

I was reading the Editorial of Climatic Change this morning, and am intruiged by the hypothesis that it was temperature which drove the encephalisation of hominins (as well as several other key evolutionary events like the emergence of phototrophs, eucaryotes and Metazoa). They note that lower temperatures encourage the evolution of larger brains by improving the efficiency of thermoregulation and removing constraints on the size of energy-intensive organs. During mammalian evolution, moreover, suitable temperatures (below the threshhold for brain growth but above the minimum required for survival) appear only during the last 500 million years, incidentally also the period in which encephalisation is obvious in hominins, birds and toothed mammals.

Apparently, this hypothesis - and variants which emphasise temperature variability rather than specific shifts - have been around for some years, but the current version differs in attempting to use temperature drops to explain specific saltations in hominin evolution rather than just the general increase observable over the past few million years. Specifically, they identify both geographic regions and temporal periods which might be expected to lift the thermal constraints on encephalisation and allow hominin brain expansion, including South Africa and the higher elevations in the Rift Valley at all times and glacial periods in the late Pliocene and early Pleistocene.

The most intruiging of these suggestions for me is that the emergence of Homo erectus, itself the first hominin of relatively "modern" appearance, might have coincided with a glacial period in which temperatures fell dramatically. The emergence of Homo erectus remains one of the least well-explained major events in hominin evolution, and although temperature change is a possibility, it would be interesting to see more hypotheses. That Homo habilis might also have emerged as a result of cooling is interesting, but less so, as that species is much less markedly different from earlier taxa.

I would like to see more evidence in support (or otherwise) of this argument, because it is fascinating to consider the possibility that modern human cognitive capacity emerged as a result of releasing factors which remove earlier constraints on encephalisation rather than directly in response to new selective pressures, potentially invalidating many extant hypotheses. Currently, the evidence seems to be based primarily on the correspondence between climatic and fossil records and some energetic modelling, which is interesting but remains inconclusive. I'm not sure how these hypotheses can be demonstrated to be correct, but their exploration should prove interesting on several fronts in palaeoanthropology and cognitive science.

I suppose this may mean that in the future advocates of reducing anthropogenic carbon dioxide can cite the risk of affecting the intelligence of future generations to support their arguments...

References:

Schwartzman, D., Middendorf, G. and Armour-Chelu, M. 2009. Was climate the prime releaser for encephalization? An editorial comment. Climatic Change volume 95, pages 439-447.

The death of the "Savannah Hypothesis"?

There are many competing ideas about what caused our lineage to diverge from that leading to our closest relatives, the chimpanzees and bonobos, but the savannah hypothesis is perhaps still the best-known. It was proposed for the first time in 1925, when Raymond Dart wrote that:

"For the production of man a different apprenticeship was needed to sharpen the wits and quicken the higher manifestations of intellect - a more open veldt country where competition was keener between swiftness and stealth, and where adroitness of thinking and movement played a preponderating role in the preservation of the species."

At the time, the first australopithecine (a member of Australopithecus africanus), had just been discovered in South Africa but almost no other record of human evolution existed; Dart's ideas about the environments associated with the emergence of humans were some of the first to be articulated. Since then, although the savannah hypothesis has not disappeared from palaeoanthropology, it has been challenged by a variety of other ideas. These new ideas increasingly emphasise the importance of increased environmental variability, rather than simple shifts in external conditions like a change from wooded to savannah environments.

In addition, recent discoveries of the earliest hominins have suggested that savannah environments were not important to our evolution after all. Ardipithecus ramidus, for example, the most complete early hominin known to science (and one of the biggest news stories of 2009 when it was published in a special edition of Science) is fairly conclusively associated with woodland and forest patches. Sahelanthropus tchadensis, in contrast, is found in gallery forest near - but not inside - an area of savannah, but whether this environment represented the habitat in which Sahelanthropus lived or just that in which it died is not clear.

In light of these recent finds and changing theoretical perspectives, the savannah hypothesis seems to be persisting primarily through disciplinary inertia. So, I would like to know: does the savannah model still, implicitly or otherwise, inspire palaeoanthropological research, or do we retain the hypothesis in our papers as a mark of respect for the ideas of past masters?

References:

Dart, R. 1925. Australopithecus africanus: the man-ape of South Africa. Nature volume 115, pages 195-199.

White, T. D., Asfaw, B., Beyene, Y., Haile-Selassie, Y., Lovejoy, C. O., Suwa, G. & WoldeGabriel, G. 2009. Ardipithecus ramidus and the paleobiology of early hominids. Science, volume 326, pages 64-86.

Vignaud, P., Duringer, P., Mackaye, H. T., Likius, A., Blondel, C., Boisserie, J.-R., de Bonis, L., Eisenmann, V., Etienne, M.-E., Geraads, D., Guy, F., Lehmann, T., Lihoreau, F., Lopez-Martinez, N., Mourer-Chauvire, C., Otero, O., Rage, J.-C., Schuster, M., Viriot, L., Zazzo, A. & Brunet, M. 2002. Geology and palaeontology of the Upper Miocene Toros-Menalla hominid locality, Chad. Nature, volume 418, pages 152-155.

About me and my research

Hello! My name is Isabelle Winder, and I am a PhD student at the University of York, in the department of Archaeology. I am originally from Cambridge, and have a BSc in physical geography and an MSc in palaeoanthropology, both from the University of Sheffield. I find both subjects fascinating, and have chosen to continue to a research career in order to continue learning about them. This blog is my way of sharing those interests with any interested parties; it will contain comments on the big stories (new and old), reviews of articles and books that I read, and my thoughts about new finds and theories.

I am currently working on a project that develops a new approach to the "Neanderthal Problem" through a holistic analysis of Neanderthal and modern human morphological differentiation, niche overlap and landscape exploitation. This project is supervised by Prof. Geoff Bailey, who recently won the Antiquity prize for a joint paper on the importance of dynamic landscapes to hominin evolution. More generally, though, I am interested in hominin evolution and ecology, landscape reconstruction and exploitation by various species, human and comparative anatomy and primate evolution and adaptation, as well as physical geography and earth science.

I am a student member of the Paleoanthropology society and the Primate Society of Great Britain.