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.
Sunday 28 February 2010
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
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
Saturday 27 February 2010
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
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
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
Saturday 13 February 2010
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.
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.
Wednesday 3 February 2010
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.
------------------------------------------------------------------------------------------
The statement by the International Union of Geological Sciences re-defining the Pleistocene can be found here:
http://bit.ly/cqWHgn
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.
------------------------------------------------------------------------------------------
The statement by the International Union of Geological Sciences re-defining the Pleistocene can be found here:
http://bit.ly/cqWHgn
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