Strange Animal Models of Human Evolution
What do sea otters, wolves and capuchin monkeys reveal about our hominid ancestors?
Fossils are the clues researchers study to better understand the history of life on earth. But to interpret those clues, scientists need to consider living animals. By looking at how the bones and physiology of modern creatures correlate with walking, eating, socializing and other habits, we can make inferences about what extinct animals with similar features might have been like.
In human evolution, hominids are most often compared to their living descendants—us. They are also compared to our closest living relative, the chimpanzee. This makes a lot of sense. We diverged from the chimpanzee lineage roughly seven million years ago; we share a lot of traits because we share a long evolutionary history.
But sometimes it’s more informative to compare hominids with more distantly related species that share traits due to convergent evolution—when two species evolve analogous characteristics, not because of common ancestry, but because of similar evolutionary pressures. The wings of bats and birds are one example; the fins of dolphins and sharks are another. Here are a few of my favorite examples of unexpected species that have played a role in the study of human evolution.
Sea Otters: These marine mammals don’t appear to have much in common with hominids, until they open their mouths. Sea otters have molars that resemble those of the genus Paranthropus, known for its giant jaw, massive chewing muscles and large molars with round cusps. Sea otters eat a lot of different foods, including critters with hard shells or outer skeletons; they can pop a clam, snail, sea urchin or crab into their mouths and crunch it whole. Researchers have long thought Paranthropus must have also eaten hard objects, perhaps nuts and seeds, in part because of its similarities with sea otters. Although recent research indicates these hominids may have spent much of their time grazing on tough plants such as grasses, rather than eating nuts, paleoanthropologists continue to study sea otters to see what they can learn about Paranthropus and other hominids.
Wolves: Wolves often come up in studies of human evolution, usually in discussions of dog domestication. But the social carnivore is useful in other ways. Adam Clark Arcadi, an anthropologist at Cornell University, used wolves to examine how many species of Homo might have had existed at one time. The question arises in relation to modern humans and Neanderthals: Were Neanderthals a separate species or just a subspecies of Homo sapiens? According to Arcadi, it’s likely there was only one human species. Even though regional populations might have developed different physical traits, a united species would have been maintained as long as there was some migration and mating between populations, what scientists call gene flow. Because humans are wide ranging and can live in a variety of habitats, he says, it’s likely gene flow was sustained.
As a way to think about the problem, Arcadi looked for another type of animal that is also wide-ranging and tolerant of numerous habitats—the wolf. Wolf packs can travel more than 100 miles per day; they can survive in deserts, forests, tundra and even urban areas; and they eat animals as big as caribou and as small as rodents, even munching on fruits or grass if they have to. The wolf analogy supports Arcadi’s case: The gray wolf, for example, traditionally lived throughout all of North America, Europe and Asia (before humans got in the way), yet it remained one species, Canis lupus. If the gray wolf can stay just one species, with about ten regional subspecies, Arcadi argues, then it’s also possible that there was just one species of Homo during the days of Neanderthals and modern humans.
Capuchin Monkeys: Unlike sea otters and wolves, capuchin monkeys may not seem like an unusual animal to compare hominids to. Yet in the primate world, more than 35 million years of evolution separate humans and capuchins. What they have in common are big brains and tool use. In Brazil, some populations of capuchins use sticks to probe holes and stones to hammer open palm nuts. Some researchers think we can learn more about how and why tool use evolved in hominids by exploring the differences between capuchin populations that use tools and those that don’t. One difference, noted by Eduardo Ottoni and Patricía Ozar of the University of São Paulo, Brazil (PDF), is the tool-using capuchins tend to be more terrestrial, living in savanna-like environments. Studying differences between tool-using and non-tool-using capuchins may also shed light on how tools affect social behavior.
Palm nuts must be a very nutritious and rewarding snack for the monkeys, because nutcracking appears to be very laborious. The cat-sized monkeys must lift what are to them boulder-size rocks up almost over their head and then pound them down on the nuts. The best way to appreciate a capuchin’s determination and skill is to watch one in action.