Photograph by Carl Posey
Anthropologist Robert Martin is provost of academic affairs and curator of biological anthropology at the Field Museum in Chicago. He has devoted his life to studying the biology and evolution of primates as a basis for understanding human origins. His textbook, Primate Origins and Evolution, established standard thinking in the discipline. But his latest thinking is that humans, as well as other mammals and birds, evolved far earlier than previously thought.
Why even bother to study evolution?
M: A lot of people say, “Well, what’s the point, you know? You reconstruct the evolutionary tree for primates and humans, but so what?” And my justification for that is it’s like history. In the same way that we’re interested in our cultural history, looking at our biological history is a valuable thing to do. It helps us understand human society today by giving us parameters to evaluate our biological and cultural selves. For example, it can tell us that race and gender differences in brain size are unrelated to intelligence.
So when did human life begin?
M: A better question is: When did the line leading to all modern primates diverge from other mammals? The classic story is that all modern mammal groups started to develop no more than 65 million years ago, after the end of the Cretaceous Period, when a probable meteorite impact led to a major extinction in which all the dinosaurs died. The standard explanation is that the evolution of the modern groups of mammals and birds didn’t get under way until after that. If you go to a paleontologist and ask, “When do you think the primates started?” the paleontologist will say, “Well, the earliest fossil we can find that’s definitely a primate is 55 million years old. So if we add a few million years as a safety margin, we can assume that primates evolved about 60 million years ago.”
But you say that’s wrong?
M: I was looking at bats, and it occurred to me that when you first find bats 55 million years ago, they look just like modern bats. All their key characteristics are developed. And so I suspected that things must go back further than that, unless you just believe they somehow popped into existence. So I got together with a mathematician and two graduate students, and we developed a model where we take the number of living species of primates along with all the fossil forms in time slices, and then repeatedly fit randomly branching trees following certain basic rules to estimate when the common ancestor emerged. And the answer is that the common ancestor of primates probably appeared about 20 million years earlier than people thought. So it’s closer to 85, not 65, million years ago.
Anthropologists are fond of pointing out that, despite what we see in the movies, dinosaurs and humans did not exist at the same time. But you’re saying primates were running around in the age of dinosaurs?
M: I’m not sure whether they were actually running around alongside the dinosaurs. All we can say is that they existed at the same time. But they may not necessarily have occurred in the same areas. There have been suggestions that modern mammals and birds evolved in upland areas, cooler areas, and that’s why birds and mammals developed control of their body temperatures. So it’s possible that these early relatives of modern mammals evolved in cooler, upland areas and that the dinosaurs were in the hotter, lowland areas. So they probably were around at the same time, but they didn’t necessarily see each other.
What did that first primate look like?
M: Our suggestion is that 85 million years ago, you had a creature weighing about two pounds that was tree living, with grasping hands and feet, large, forward-facing eyes, and probably a relatively big brain. In short, it looked like a modern primate. In recent years, molecular biologists have produced evidence that primates diverged from other mammals 90 million years ago and began to diversify at least 80 million years ago, so both of us are saying the same thing: You have a recognizable primate well back in the Cretaceous Period.
Why does 20 million years matter?
M: It suggests that primates could have originated in the landmass composed of India and Madagascar, not in Africa. At least 130 million years ago, Indo-Madagascar separated from Africa. More than 40 million years later, Madagascar broke off from India, which continued on its merry way until it collided with Asia about 60 million years ago. A few million years after that, fossil primates abruptly appear in Asia, Europe, and North America. There’s an emerging theory called the Indian ark hypothesis that says India carried with it a whole bunch of plants and animals. Primates couldn’t have been part of this if they evolved as late as is believed. I am looking for Cretaceous primate fossils in India to test my hypothesis.
When did we split from apes?
M: The time of divergence between humans and our African great ape cousins, the chimp and the gorilla, has been calculated using the known fossil record. The date of 5 million years ago has almost become set in stone. I reckon that we diverged at least 8 million years ago.
Why is that important?
M: There has been a huge controversy over whether the Neanderthals were part of the European origins of humans or whether the Neanderthals were a separate lineage and, indeed, a separate species. I think that’s been resolved now partly because of work I carried out on the Neanderthal skull with colleagues at the University of Zürich. I’m convinced that Neanderthals were a totally separate lineage. But even people who accept that Neanderthals belonged to a separate lineage believe that they diverged about 600,000 years ago. I think the time of divergence is closer to a million years. If Neanderthals branched away a million years ago, a lot of their characteristics evolved independently from ours.
M: Most notably, their larger brains. It seems unbelievable, but on average Neanderthals actually had larger brains than modern humans. In fact, at that time our own direct relatives also had larger brains. This is because both they and the Neanderthals had bigger bodies. Our brains have actually been decreasing in size for 30,000 years. More important, early divergence would mean that a significant part of brain expansion in Neanderthals took place completely separately from that in our own lineage. Parallel expansion of the brain has happened elsewhere. For example, you’d think that the primate with the next biggest brain relative to body would be a close relative—a chimp or a gorilla. But it’s not. It’s the capuchin monkey. They’re the little fellows that organ-grinders carried around because they were good at doing tricks . . . because they are very, very smart.
So how did capuchins, and for that matter, humans, become so smart?
M: Brains are unusual organs in that most of their growth occurs early on. Most human brain development happens in the womb and during suckling. By the age of 5 or so, that’s pretty well it. So most of the resources for brain growth are provided during pregnancy and nursing. Our ancestors very likely suckled for about four years. So we are what we eat. It’s a cliché, but brain size has everything to do with diet. Capuchins must have at some point adopted a very high-energy diet because their digestive system looks a lot like ours.
What else can you tell from looking at the behavior of other modern primates?
M: Comparative studies let us establish the rules of the game. For example, we can observe that when males are significantly bigger than females in modern primates, males are likely to live in groups with breeding access to several females. On the other hand, in groups with monogamous relationships, males and females are typically very similar in size.
But human males are typically larger than females. Does that mean they’re not likely to be monogamous?
M: That’s right, although it gets complicated because the bigger a species is, the more likely you’ll get some degree of size difference between males and females. I nevertheless think there’s a greater degree of sexual dimorphism than you would expect if humans were biologically adapted for a monogamous system. I would expect men and women to be much more similar in body size if we were biologically adapted for monogamy.
Are there other markers for monogamy?
M: There are markers that indicate whether the mating system involves several males or just one male, which includes monogamy. If you look at the size of the testes relative to body size, species in which the males have particularly large testes are found to have a multimale system accompanied by sperm competition. If you look at the human male, our testes actually fit into the single-male category, which includes both monogamy and polygynous harem systems. The same result emerges when you look at the size of the midpiece in primate sperm.
What about the idea that there’s a missing link between humans and other apes?
M: If you accept the 5-million-year divergence date, then you might believe that we’ve discovered practically everything. We have Australopithecus species, such as Lucy, and some fossil members of that lineage go back over 4 million years. So you might conclude that Australopithecus is the missing link. But if you believe the divergence occurred around 8 million years ago, we’ve got a lot of fossils yet to find.
So the missing link is still missing?
M: There are a lot of missing links. But finding the divergence between apes and ourselves is getting more promising. One of the most spectacular recent discoveries was made in Chad by a French team that turned up a 6- to 7-million-year-old skull. It’s a surprisingly small skull, but it shows some intriguing similarities to humans. If you buy into the 5-million-year date, then it can’t possibly be a hominid because we didn’t split from the chimpanzees until after that. But if you go with our revised 8-million-year estimate, it could well be the missing link. At least the possibility is left open that this could be a very early member of the lineage that led to us.
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