TCS Daily

Your Presence Is Requested

By Sallie Baliunas - August 12, 2002 12:00 AM

Given the total, absolute, and final disappearance of Homo sapiens, then not only would the Earth's community of Life continue to exist, but in all probability, its well-being would be enhanced. Our presence, in short, is not needed.
--Paul Taylor, Respect for Nature: A Theory of Environmental Ethics, 1986, Princeton University Press, p. 115.

What a silly sentiment - that without Homo sapiens, the biosphere would flourish. It contradicts the scientific facts of biology. It suggests that the existence of Homo sapiens is a fluke, separate from nature. But Homo sapiens is as much a part of nature as amoebae or trees. To deny that fact is to deny biology.

The origin of Homo sapiens is explained by evolution, the most significant concept in biology. Charles Darwin first expressed the theory in his 1859 book, On the Origins of Species by Means of Natural Selection. By 1871 in The Descent of Man, Darwin had considered that humans and apes were so anatomically similar that they must have had a common ancestor, who probably appeared first in Africa.

In 1924 Raymond Dart recognized the skull of a child called the "Taung Baby" found in South Africa as that of an upright-walking child with a relatively small brain case. The child was probably less than five years old at death, because his adult teeth are still undescended in the skull. Dart called this new species Australopithecus africanus, which lived from before three million to about one million years ago and represents one member of the hominids. A hominid is any species more closely resembling Homo sapiens than the chimpanzee, our nearest extant evolutionary relative. Homo sapiens is the only living member of the hominid clade, or group. The other members are extinct.

Since 1925, anthropologists have found a wealth of hominids, with the most ancient from Africa. A few of the hominids that have been discovered include Homo erectus (living between 2 million and 400,000 years ago), Homo habilis (about 2.2 to 1.6 million years ago), Australopithecus afarensis (3.9 to 3 million years ago; Lucy is the familiar example, whose skeleton was discovered by Donald Johanson in Hadar in 1974), Kenyanthropus platyops (3.5 million years ago) and Orrorin tungenesis (6 million years ago).

Researchers from Poitiers, France, just added a new hominid to the group. The evidence from six specimens portends a new species, Sahelanthropus tchadensis, dating from approximately 6 to 7 million years ago. Sahelanthropus has a small braincase - about one-third the volume of the average modern human and similar in volume to a modern chimp's braincase - but walked upright and has a relatively unprotruding chin, making it less chimp-like and more human.

The age of Sahelanthropus is astonishing. Mapping the rate of change in DNA between humans and chimps, researchers estimate that humans and chimps split from their common ancestor around 5 to 7 million years ago. The speculation is that Sahelanthropus may be a hominid that had just diverged from our common ancestry with chimps.

Sahelanthropus may be related but not a direct ancestor to Homo sapiens. How the different hominids relate to each other is deep-frontier anthropology. But one fact emerges from the richness of hominids: Upright-walking, two-legged creatures with increasing thinking capacity blossomed in the last few million years, with Homo sapiens as the latest and brainiest in the group. With intelligence comes flexibility to respond to changing environmental conditions, an important survival advantage. For example, Homo sapiens is flexible enough to thrive in a wide range of climates.

A complex and massive brain, while useful for survival, comes with a price. A large brain is metabolically costly, and ours requires about 20% of our caloric intake for energy support, although the brain is 2% of body weight. And the brain needs climate control, thus requiring a warm-blooded body in which to house it. That development took an incredibly long time on earth - some 3.5 billion years of biological evolution. To some researchers the long lead-time before the rise of braininess suggests that intelligence is an oddity - not my view, but that is another story.

With the wave of new hominid finds, Johanson, the discoverer of the A. Afarensis skeleton Lucy, imagines the hominid family tree to be "bushy" or leafy. That image emphasizes the multiplicity of proto-intelligent, tool-making, bipedal species that appeared and evolved rapidly over the past few million years. The proliferation of such species suggests that an environmental niche was filling rapidly. The common traits of intelligence, toolmaking and upright, two-legged walking allowed those species to survive better the hazards of nature.

The rich hominid past says two things. First, Homo sapiens, who appeared around 250,000 years ago, will become extinct. Judging from the hominid record, Homo sapiens will give way to something brainier, perhaps in 500,000 years. That is a fact of biology.

Second, should Homo sapiens go extinct before our successor species arrives, within a few million years another very brainy creature would arise to fill the empty environmental niche. The new species' forebear may be the modern chimps. Far from being unneeded, Homo sapiens or an equivalently intelligent species is deemed necessary by nature. That, too, is a fact of biology.



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