As 2012 nears its end, one thing stands out as the major theme in human evolution research this year: Our hominid ancestors were more diverse than scientists had ever imagined. Over the past 12 months, researchers have found clues indicating that throughout most of hominids’ seven-million-year history, numerous species with a range of adaptations lived at any given time. Here are my top picks for the most important discoveries this year.
1. Fossil foot reveals Lucy wasn’t alone: Lucy’s species, Australopithecus afarensis, lived roughly 3.0 million to 3.9 million years ago. So when researchers unearthed eight 3.4-million-year-old hominid foot bones in Ethiopia, they expected the fossils to belong to Lucy’s kind. The bones do indicate the creature walked upright on two legs, but the foot had an opposable big toe useful for grasping and climbing. That’s not something you see in A. afarensis feet. The researchers who analyzed the foot say it does resemble that of the 4.4-million-year-old Ardipithecus ramidus, suggesting that some type of Ardipithecus species may have been Lucy’s neighbor. But based on such few bones, it’s too soon to know what to call this species.
2. Multiple species of early Homo lived in Africa: Since the 1970s, anthropologists have debated how many species of Homo lived about two million years ago after the genus appeared in Africa. Some researchers think there were two species: Homo habilis and Homo rudolfensis; others say there was just H. habilis, a species with a lot of physical variation. It’s been a hard question to address because there’s only one well-preserved fossil, a partial skull, of the proposed species H. rudolfensis. In August, researchers working in Kenya announced they had found a lower jaw that fits with the previously found partial skull of H. rudolfensis. The new jaw doesn’t match the jaws of H. habilis, so the team concluded there must have been at least two species of Homo present.
Perhaps no other human trait is as variable as human height. At 5’4″, I’d be dwarfed standing next to 6’3″ Kerri Walsh, the 2012 Olympic gold medalist in beach volleyball. But next to an African pygmy woman, I’d be a giant. The source of that variation is something that anthropologists have been trying to root out for decades. Diet, climate and environment are frequently linked to height differences across human populations.
More recently, researchers have implicated another factor: mortality rate. In a new study in the journal Current Anthropology, Andrea Bamberg Migliano and Myrtille Guillon, both of the University College London, make the case that people living in populations with low life expectancies don’t grow as tall as people living in groups with longer life spans. They also argue changes in mortality rates might account for the jump in body size from Australopithecus to Homo some 2 million years ago.
From an evolutionary standpoint, Migliano and Guillon note, it’s beneficial to start reproducing as soon as possible if you live in a society where individuals typically die young. That way you can have as many babies as possible in a short amount of time. Thus, you should stop growing relatively early in life and start devoting your energy to having children and taking care of them. Having a shorter developmental period means you can’t grow as tall, on average, as someone who has more time to mature. But getting big has reproductive benefits: Larger individuals tend to take in more energy and therefore can invest more energy in reproducing. So in societies with lower mortality rates, and longer adulthoods, it’s better to mature slowly and grow bigger and taller. Over time, populations experiencing different mortality rates will adapt to have shorter or longer developmental periods—and therefore be shorter or taller. (Of course, there is also variation within a population. But here, and throughout the post, I’m talking about population averages.)