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.
