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Chip Walter's new book asks why only one particular variety of human survived prehistory. 

Last Ape Standing suggests our longer childhoods gave us the advantage.

The past isn't what it used to be. Especially in paleoanthropology, a discipline constantly adapting to new fossil finds and, lately, improved DNA testing. Things change this fast: Chip Walter's engaging new book, Last Ape Standing (Walker and Company), is based largely on research released just since publication of his 2007 book about human evolution, Thumbs, Toes and Tears.

If your knowledge of how humans developed was gleaned any earlier than that, Last Ape Standing holds some surprises. For one, the number of known human species is up to 27 — not just early humans like australopithecus afarensis (including the famous "Lucy") and homo heidelbergensis, and later beings like Neanderthals, but several discovered while Walter was researching this book. "Clearly, there were all kinds of humans all over the planet," says Walter in an interview. Meanwhile, species once regarded as our direct ancestors, such as homo habilis, are now considered dead branches on the family tree.

What's more, scientists say, after our species arose 200,000 years ago, we actually shared the Earth with several other human species; some of them even mated with us. And that interbreeding happened largely during the past 50,000 years or so, after homo sapiens survived a catastrophic event that reduced their numbers to as few as 2,000 individuals in Africa.

Walter, of Squirrel Hill, is a documentary filmmaker, former CNN bureau chief and veteran science writer who runs the entertaining, anthropology-themed website allthingshuman.net. The more he learned about other human species — all of them clever, some with brains bigger than ours — the more he wondered, "What made us a success?"

"Success," of course, is conditional: Did homo erectus, who lasted more than 1.5 million years, "fail"? After all, if we manage to survive another hard-to-imagine 5,000 years, in anthropological terms it will barely extend our tenure.

Still, we outlasted Neanderthals, whose number and range once far exceeded ours. We've also outlived such recent discoveries as the Denisovans of Siberia, the diminutive "hobbits" (homo floresiensis) of Indonesia, and China's Red Deer Cave People.

The likely reason, Walter learned by combing through hundreds of scientific papers, has a certain irony. Some 2 million years ago, climate change shrank the African rainforest, and to survive on the growing savannah, some primates evolved to walk upright. Physiologically, that required narrower hips, which meant a narrower birth canal. But brains were getting bigger, too.

To compensate, we effectively evolved for premature birth. "If we were to be born as mature as a baby gorilla is, we would have to be in the womb 20 months," Walter says. "That would clearly be unacceptable to your mother." Instead, "[w]e're these fetal apes. We can't hold our heads up, we can't see very well, we need to be fed." The upside: "The vast majority of our brain development ... takes place outside the womb."

Childhood for homo sapiens was even longer than those of other humans. That made possible more experiential learning, and therefore more cultural development: fire, better tools — even the social structures required to protect helpless human infants. The survival challenge of "premature" birth spurred our greatest assets.

Homo sapiens, geneticists say, nearly went extinct about 75,000 years ago, following a volcanic eruption or other catastrophe. But we survived, and rapidly spread into Europe, Asia and beyond. It's fun to speculate, as Walter does in Last Ape, on how encounters between the different human species we met there went. But Walter says that the idea that homo sapiens simply wiped out Neanderthals and other human species is misguided.

"It's really just that the world was really tough, and was hammering on all of them," he says. "And we, just by the luck of genetics and evolution, developed these tools largely because of our long childhood that made us better at adapting. We became the creature that could adapt the landscape to us, rather than a creature that had to adapt to the landscape."

But before outliving certain other species, we hooked up. In 2010, Germany's Max Planck Society announced that DNA evidence suggests that Neanderthals and homo sapiens interbred — and that most non-Africans today have a little Neanderthal in them. "That blew everybody's mind," says Walter. "That's where some of our red hair and freckles come from."

Last Ape Standing also cites DNA evidence suggesting homo sapiens bred with homo erectus — perhaps as recently as 25,000 years ago, or 50,000 years after erectus was thought to have gone extinct.

Like Thumbs, Toes and Tears, Last Ape concludes by asking where human evolution is bound. With technology accelerating, says Walter, "We've created this planet we're becoming aliens in. ... How do we adapt?"

 "We're going to morph into some other creature, but I'm not sure we'll even be aware of it when it happens," he says. "I don't think a Neanderthal was necessarily aware that it was going to happen when they switched over from being homo heidelbergensis. ... But at some moment, enough changes that you are a new species."

This time, he speculates, nanotechnology and genetic engineering will allow us to control that evolution: "It's going to be a wild ride." 

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