Laelaps

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A simplified evolutionary tree of primate relationships showing the placement of Darwinius in relationship to other groups. From Williams et al., 2010.

ResearchBlogging.org

The study of human origins can be a paradoxical thing. We know that we evolved from ancestral apes (and, in fact, are just one peculiar kind of ape), yet we are obsessed with the features that distinguish us from our close relatives. The “big questions” in evolutionary anthropology, from why we stand upright to how our brains became so large, are all centered around distancing us from a prehistoric ape baseline. Despite our preoccupation with “human uniqueness”, however, many of our traits are extremely ancient, and they can be traced back much further than the seven million years or so that hominins have existed.

As acknowledged by paleontologists Blythe Williams, Richard Kay, and Christopher Kirk (who confirmed that Darwinius was only a very distant relative of ours last week) in a new PNAS paper, “human evolution did not begin 6-8 million years ago with the phylogenetic split between the chimpanzee and human lineages.” It is not as if the first hominins appeared out of nothing and began an upward march to us. Instead we know that we could hypothetically trace our lineage all the way back to the last common ancestor of all life on earth, and any point we chose to stop along that “unbroken thread” could tell us quite a bit about our history. In the case of the present review, Williams, Kay, and Kirk pick up with the origin of anthropoid primates.

The origin of anthropoid primates, the group to which monkeys and apes belong, has long been a controversial topic among paleontologists. The past forty years, especially, have been marked by increased discussion and debate on the subject, and it has only been recently that scientists have been able to resolve some of the long-running disputes.

Some time before 55 million years ago there was a divergence which formed the two great branches of the primate family tree. On the one side there were the haplorrhines, represented today by tarsiers and anthropoids, and on the other were the strepsirrhines, the group to which living lemurs, lorises, and bush babies belong. On this much everyone was agreed. The trouble was parsing these relationships among fossil primates and determining which group was most closely related to the first anthropoids.

Some researchers proposed that fossil tarsiers and a closely related, but extinct, group called omomyids were the best candidates for anthropoid ancestors, while others thought that the lemur-like adapiformes (such as Darwinius) were even closer. For years the debates continued to fill up journal pages and symposium slots, but, as in other subfields in paleontology, resolution would eventually come through an interdisciplinary approach. Through a combination of genetic, zoological, and paleontological data scientists have been able to determine that tarsiers and their omomyid relatives were most closely related to early anthropoids (with Darwinius and its kin being more closely related to lemurs).

But resolving these large-scale relationships has only been one part of the ongoing debate over anthropoid origins. New discoveries have also altered our understanding of what early anthropoid primates were like and where they lived. Paleontologists have found at least 15 species of fossil anthropoids spanning the 30-37 million year old range in the Fayum depression of Egypt, and a series of recent discoveries in Asia has acquainted paleontologists with a series of slightly earlier anthropoids. Altogether these primates document the radiation of early anthropoids, and they illustrate some interesting evolutionary changes.

As every vertebrate paleontologist knows teeth are the keys to understanding the mammal fossil record, and the teeth of early anthropoids show that they started out as relatively small animals that fed on insects and fruit. As some lineages became larger, however, they started to eat lower-quality foods like leaves, and this is in accord with what we see among living primates. As is well-known, small primates must rely on high-quality food to fuel their tiny bodies, but larger primates with slower metabolisms are able to subsist on lower-quality food. Size, metabolism, and diet are all closely tied together, and from the available evidence it appears that the same constraints that shape the diets of living primates also affected their prehistoric relatives.

Among the most interesting features of anthropoids, however, are their eyes. Anthropoid primates have eyes set in forward-facing orbits separated from the rest of the skull by a bony partition in the back. Strepsirrhine primates (including Darwinius) lack this bony wall, and there is another feature that easily distinguishes living strepsirrhine primates from their haplorrhine cousins. Primates such as lemurs and lorises have a structure in their eyes called the tapetum lucidum which reflects light and allows them to see better in low-light conditions. Anthropoid primates lack this structure, as do tarsiers, and so haplorrhines active at night typically have extremely large eyes to compensate. What this suggests is that both tarsiers and anthropoids evolved from a diurnal ancestor which did not need the special night-vision adaptation that the strepsirrhines have. This would explain why haplorrhines which are active at night, such as tarsiers and owl monkeys, have extremely large eyes.

The authors of the new paper review increases in early anthropoid brain size, changes in the organization of the anthropoid brain, the sense of smell in anthropoids, and other features, as well, but rather than summarize all their points here I would like to draw attention to something else. Our present understanding of anthropoid origins has emerged from interdisciplinary efforts based in paleontology, zoology, anatomy, genetics, and development. In this way the evolving debate over anthropoid origins has tracked the emergence of paleobiology, or a more synthetic type of paleontology that is much more than the marriage of geology and comparative anatomy.

There is little doubt that such approaches will continue to be productive. New fossil discoveries will help us better understand what primates were like in the distant past and the study of living primates can help us grasp how some of the changes we see in the fossil record were affected. A scientist who wants to understand the origins of primates cannot afford to only be an anatomist or paleontologist. They must instead be a carry on in the tradition of the true naturalists who tied together evidence from disaprate fields to better understand the natural world.

Williams, B., Kay, R., & Kirk, E. (2010). New perspectives on anthropoid origins Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0908320107

Comments

  1. #1 Poppy Top
    March 9, 2010

    I liked the three crosses in the diagram. Evolution sucks, but the three crosses here lifts my heart.

  2. #2 D J Wray
    March 9, 2010

    Other than the fact that these highly qualified researchers have rehashed a bunch of existing information, did they have anything to offer about the increase in brain size or the important questions of migration, consciousness or language? These are the “big questions” that continue to baffle modern science.

    D J Wray

  3. #3 Ed Yong
    March 10, 2010

    Yeah, I can’t believe that the people who wrote this review on anthropoid origins just reviewed the science on anthropoid origins. Why didn’t they also explain the origin of consciousness, or work out the origin of life, or develop an HIV vaccine, or find the Higgs boson, or work out where all the socks and ballpoint pens disappear, or give credence to my pet theory while massaging my shoulders and feeding me grapes?

    They’re just LAZY.

    I don’t need to write my name in this line. It’s down there. See?

  4. #4 Colin Bartlett
    March 10, 2010

    Weird! I’d always had Tarsiers pinned as highly derived loris-line prosimians. Given how Afrocentric primate diversity is these days, I occasionally have to remind myself that Primates are Laurasiatheres, which implies (possibly erroneously) a certain amount of evolutionary time spent in Europe, Asia, and/or North America.

    So did all the Asian primate lineages just get squashed by the Himalayas and outcompeted by African migrants? What is it with Africa and producing waves of Primate invasions into Eurasia? You’d expect, given the size of the landmasses involves, for the invasions to tend in the other direction (because animals from larger landmasses tend to outcompete animals from smaller ones, if we completely ignore corvids, elephants, and edentates).

    This is what I get spending all my time studying manatees, flamingos, and mekosuchines, I guess. Which is why I appreciate posts like this. Thanks!

  5. #5 Caleb
    March 10, 2010

    Maybe your foolish self come from an ape, which explains a lot, but for the rest of us, we were created by an intelligent desiger. Open your eyes and think for yourself.

  6. #6 Hai~Ren
    March 10, 2010

    Caleb:

    Open your eyes and think for yourself.

    Oh the irony.

    We have the evidence. You don’t.

  7. #7 Nick Gardner
    March 10, 2010

    Maybe your foolish self come from an ape, which explains a lot, but for the rest of us, we were created by an intelligent desiger. Open your eyes and think for yourself.

    Hope you got a D for copypaste trolling.
    Comment #1 deserves at least a B for creativity and comment #2 deserves a C for just being the standard why does this matter, though at least he had a point of sorts. :P

    Cheers

  8. #8 Nick Gardner
    March 10, 2010

    Brian wrote:

    The “big questions” in evolutionary anthropology, from why we stand upright to how our brains became so large, are all centered around distancing us from a prehistoric ape baseline.

    D J Wray wrote:

    Other than the fact that these highly qualified researchers have rehashed a bunch of existing information, did they have anything to offer about the increase in brain size or the important questions of migration, consciousness or language? These are the “big questions” that continue to baffle modern science.

    Answering those “big questions” requires an understanding of the ‘prehistoric ape baseline’ as Brian spoke of. Williams et al. are trying to tackle the questions around the origins of human-line primates.

    Brian’s post offers a layman’s review of the literature around this problem.

    I really don’t get your point D J Wray, that is why I dismissed your post as mere trolling in my prior comment on this blog entry.

    Cheers,
    Nick

  9. #9 A Newcomer
    March 10, 2010

    On the article, fascinating stuff. Proof positive that, unlike what some believe, science marches on, and “missing links” can easily be disproven.

    On the comments here… why is it that you never see creationists popping up on articles about the common ancestor of spiders and scorpions, but when Homo Sapiens are involved, they come right out of the woodwork?

  10. #10 Blake Stacey
    March 10, 2010

    “Evolution sucks, but the three crosses here lifts [sic] my heart.”

    Those crosses mean that those species are extinct.

  11. #11 Owlmirror
    March 10, 2010

    I liked the three crosses in the diagram. Evolution sucks, but the three crosses here lifts my heart.

    If Adapiformes and Omomyformes are the two thieves, that means that Eosimiidae are Jesus crucified between them.

    So Eosimiidae died for your sins, and/or Jesus is an extinct monkey!!!1!one!

    OK, that doesn’t make much sense, but neither do any of the other creobot comments.

  12. #12 Nick Gardner
    March 10, 2010

    Actually, Owlmirror, if you rotate the branches around, you’ll find that the Omomyiformes died for your sins, and therefore, tarsiers are possible holy descendants of monkey-Jesus.

    Behold the face of your God:

    http://akshatb.files.wordpress.com/2009/01/tarsier_2.jpg

  13. #13 Thinker
    March 11, 2010

    … tarsiers are possible holy descendants of monkey-Jesus.

    Please, please don’t let Dan Brown know…

  14. #14 johannes
    March 11, 2010

    > I occasionally have to remind myself that Primates are
    > Laurasiatheres,

    Actually, they are Euarchontogliraes aka Supraprimates. But yes, this is the sister group of Laurasitheria and together they form a clade Boreoeutheria. This strongly suggests a Laurasian origin – although perhaps on the European islands rather than on the mainland. The fossil record also supports a Laurasian origin of Primates in peculiar and Euarchontoglirans in general: Fossils of basal glirans tend to come from Asia, early archontans like plesiadapids from Europe and NA (but there is a single plesiadapid *Altiatlasius* from late Paleocene Marokko), although we should remember that the Paleocene fossil record from Africa is rather poor – the discovery of a Lagerst├Ątte might change everything. The only other well-studied animal from Palaeocene Gondwana that *might* be an Euarchontogliran is the graviportal *Carodnia* – but only if it’s related to dinoceratans and if dinoceratans are anagalids, two rather large ifs.

  15. #15 Jeremy
    March 11, 2010

    Man, it’s depressing that even a simple research review of primate evolution (I very much enjoyed btw, Brian), completely devoid of metaphysical nonsense, can descend into a self-serving clusterfuck of competing worldviews.

  16. #16 DDeden
    March 13, 2010

    Brian, to your crosses & crowns I’ll add stars & crescents (my URL & Ed’s Post: http://scienceblogs.com/notrocketscience/2010/03/subliminal_flag_shifts_political_views_and_voting_choices.php#more
    Lucky Charms anyone?

  17. #17 John
    March 31, 2010

    “As every vertebrate paleontologist knows teeth are the keys to understanding the mammal fossil record…”

    While teeth are important (especially since they are often so well preserved), keying on them can lead to lots of misconceptions IMO. Teeth seem to be more plastic from an evolutionary perspective and adapt often to the diet at hand ending up with distinct homologies that appear very much the same, but without creating an accurate clade. Teeth evolve to eat what they eat, and we know that very unrelated species eat very similar things and have very similar teeth.