Avoiding Extinction

Human success has had to surmount dangers that have been challenging primates for millions of years now, an audience was told at last weekend’s annual meeting of the American Association for the Advancement of Science held in San Francisco. In a presentation titled, “The Reduction of Hominid Species,” a stand-in for Nina Jablonski reported on the extinction episodes that have wiped out so many members of the human line. The presentation did not discuss speech origins, but explored themes that are regularly examined on this blog.

Dr. Jablonski was the subject of a memorable profile in the New York Times last January (here). She could not attend the San Francisco conference because of a last minute emergency, so her presentation was given by Mark Weiss. He used her PowerPoint slides and accompanying notes.

The most striking feature of the presentation was the way it put human evolution into a primate context. The human challenges of survival turn out to be the primate challenges writ large. I noticed  two weeks ago when I visited the new Hall of Human Origins in the American Museum of Natural History (see: Quick Tour) that the great age of primates seemed to have ended just about the time (give or take a million years) of the last common ancestor between humans and chimpanzees. Jablonski’s paper describes a mammal order facing tough times.

Primates appeared 50 to 60 million years ago in a time of especially lush conditions. The weather was consistently warm, the humidity high, the rainfall plentiful, and seasons were hardly visible. With all these nutritious foods—protein-rich leaves, fruits, and seed oils—there was enough to support an unusually luxurious life history. Longevity (relative to body size) was great, infants were born singly and came relatively late in a female’s life. Monkeys will never be anyone’s idea of a fertility symbol. The brain sizes grew, in accordance with trends discussed recently on this blog. (See:  Smarter is Fitter) Other orders of mammals have not had it so plush, but these luxuries were put to the test about 5 million years ago with the rise of the Pliocene Epoch, which brought a general cooling and the replacement of primate-rich rainforests with savanna grasslands. Starting just under two million years ago, the Pleistocene Epoch that followed the Pliocene* and lasted right through the last ice age was even more disruptive. The great age of primates was over, and yet during that same period one primate line (that would be us) has managed to expand around the globe. We have done it, says Jablonski, partly by creating a rainforest in our heads, that is by maintaining the conditions of luxurious plenty that our long-ago primate ancestors enjoyed, and partly by getting rid of some of their most extreme luxuries. In particular, we have increased our child bearing rate.

Jablonski’s slides included a chart taken from Wikipedia (entry here) showing the cooling of the earth over the last 5 million years. You can see clearly that we have gone from a period of stable warmth to cooler, unstable times.

These climate changes did not just alter the lives and ways of primates. During the Pleistocene, while humans were spreading over the earth, many mammals got bigger, storing more energy in their bodies so that they could get through periods of shortage. For primates, especially the big brained apes, however, the cost of being smart made it difficult to survive without plenty of food. Most of the apes disappeared, but a few survived in the shrinking rain forest. Deforestation, of course, continues to this day and threatens to remove the very last refuges for chimpanzees, gorillas, and orangutans. Meanwhile, the human ancestral-line evolved in the opposite direction, adapting to the demands for widespread foraging in unstable niches, moving into those newly expanding niches while the rain forest was retreating.

Each of these gross climate changes has seen widespread extinctions. For the human line of descent, these extinctions have meant survival for the species that

• could travel the longest distances,
• exploit the widest range of food resources,
• and overcome the “slow” life history parameters of  primates.

Jablonski said nothing in her slides about the contribution of speech to this struggle. I will save my own comments on this issue for tomorrow’s post.

The first of the great extinctions among the bipedal apes erased the Australopithecines (and many other monkey species who could not expand their forage areas widely enough). They were survived by Homo species who evolved “a functionally naked skin with enhanced sweating abilities.” This development gave them the ability to forage for greater lengths of time over a wider range.

The second great extinction was among the Homo erectus lines in Asia and Europe. (The European line was the Neandertal.) The erectus line in Asia went extinct without facing competitive pressures from the new Homo sapiens model. They were smart and had diverse tools, but apparently were not spry enough to find sufficient food, especially for their young, to survive through the hard seasons.

Jablonski’s presentation did not discuss the “bottleneck” that Homo sapiens also passed through at about the same time the Asian erectus was disappearing, but apparently we got through this period only by the skin of our teeth. Yet we did get through, outlasting the Neaderthals by using a “slightly smaller body, slightly smaller brain, and slower growth rates.” This outcome looks surprising. After two million years of success going to the bigger-brained individual, there seems to have been a slight retreat. That sort of thing provides a clue for tomorrow’s discussion linking this material with speech origins.

* A mnemonic for remembering the Pliocene-Pleistocene order: After the PliOHcene came the PleisTWOcene.

Comments

[Dr. Leonid Gavrilov, Ph.D.]

Please delete my earlier mistaken comment above.

Instead I was going to say this:

Thank you for your interesting post!
I thought perhaps you may also find this related post interesting to you:
Longevity Science: Evolution of Aging
http://longevity-science.blogspot.com/2007/03/evolution-of-aging.html