Evolutionary history of early primates places human origins in context

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