Human Evolution

In 1871, Darwin made it clear in “The Descent of Man” that his argument about descent with modification applied to humans as well as other organisms.

He faced much bitterness. That bitterness remains today.

There is much hard evidence detailing the evolutionary history of humans. Even for those who are comfortable with the idea of human evolution, there is much misunderstanding. Many see the evolution of humans as the “end product”, the pinnacle of evolution.

Until advances of recent years, the same processes that govern evolution in other species have applied to humans.

New Ethiopian Fossils Are From 6-million-year- old Hominid Living Just After Split From Chimpanzees Science DailyScience Daily — BERKELEY Paleoanthropologists from the University of California, Berkeley, and the Cleveland Museum of Natural History have found more fossils of a nearly 6-million-year-old human ancestor first reported three years ago, cementing its importance as the earliest hominid to appear after the human line diverged from the line leading to modern chimpanzees. Routinely, paleontologists report the discovery of new hominid fossils. The central question often revolves around whether or not the fossil lies in the line leading to modern humans and, if so, where.

Early hominid sites  The Great Rift Valley of East Africa holds a local exposure of rocks that has yielded remarkable fossils from the early part of hominid evolution. Sites in the Great Rift Valley, especially at Olduvai Gorge, hold an exposure of rocks at the needed time horizon (several million years ago). Fossil preservation was favorable, and the site is accessible (sort of) to anthropologists.

Let’s get this out of the way now…. Humans are not “at the top”. Every other species has been just as long in the making as we have. Evolution is not a ladder, it’s a tree.

The second thing, perhaps more troublesome, is that humans are not an inevitability of evolution. If we let the course of evolution play out again, it almost certainly would not play out the same way

The idea is a familiar one. Frank Capra explored it in his classic film “It’s a Wonderful Life”. In it, George Bailey is shown how life would have progressed if he had never been born. The same principle applies in evolutionary thought. The mammalian line could easily have gone extinct during the Mesozoic Era. If it had, what would have happened….

Intelligent, bipedal reptiles? Probably not, but we have no way of knowing.

Primates The mammalian order to which we belong evolved about 65 million years ago, about the same time that the last of the dinosaurs were disappearing.

Primate Characteristics Brachiation Nails Stereoscopic vision Large brain

About 40 million years ago, the early primates split into two lineages – the prosimians and the anthropoids.

FIGURE 14.5 Primate Relationships  Primates divide traditionally into two major groups. The prosimians tend to be small and nocturnal, including bush babies of Africa, the lemurs of Madagascar, and lorises and tarsiers of Southeast Asia. The anthropoids are more derived, including monkeys plus gibbons, apes, and hominids.

Today’s prosimians (basal primates) include tarsiers and lemurs. They feed on plant parts, and have largely nocturnal lifestyles. Lemur Tarsier

The anthropoids, or derived primates, include the monkeys, apes, and humans.

One group of anthropoids are the platyrhines. They evolved into the New World monkeys. They are characterized by flat, spread nostrils and (usually) a prehensile tail.

Another group of anthropoids, the catarrhines, gave rise in Africa to the Old World monkeys. In them, the nostrils are closer together and the tail, if present, is not prehensile.

In addition to the Old World monkeys, the catarrhines gave rise to the hominoids. The hominoids include the gibbons and the apes, including man.

Table 14.1 Classification of Primates

At some point, the hominid line diverged from the line leading toward chimpanzees. Over recent years, the date has been moved steadily further back in time. Note that your book says about 7 million years ago, as compared to ~5.5 million years as depicted in this figure.

One line, the robust line, became extinct about a million years ago. The other, the gracile line, leads to us.

In 1924, an Australian anatomist named Raymond Dart announced the discovery of a small skull in Taung, South Africa. The skull was of a child about six years old. Certain features suggested that the child walked upright. The skull was designated as Australopithecus africanus. Raymond Dart with the Taung skull. Dart was confident that Australopithecus was related to the ancestral stock of humans. This was fortified by findings in the 1930s.

The australopithecines were short, four to five feet in height, with a small ape-sized brain (cranial capacity range of 450 to 600 cubic centimeters). However, they stood upright, walked bipedally, and lived on the ground.

Current evidence favors the idea that there were several species of australopithecines. The light-jawed, slender A. africanus likely coexisted with a heavier-jawed, larger species known as A. robustus. It is thought that A. robustus remained exclusively vegetarian, while A. africanus became increasingly omnivorous.

In 1973 Donald Johanson, curator of the Cleveland Museum of Natural History, discovered the remains of a hominid in the Hadar region of Ethiopia. The remains have been dated between 2.6 and 3.3 mya. The most complete adult skeleton has been named “Lucy”. It has been classified as Australopithecus afarensis, and is thought to be ancestral to the earliest members of the genus Homo.

Johanson and his co-workers named the find “Lucy” after the Beatles song, Lucy in the Sky with Diamonds.

The earliest hominids, from Africa, date from 6- 7 million years ago. Sahelanthropus Orrorin

Ardipithecus ramidus lived later, about 4.5 million years ago. It was also found in Africa. At this point, we are probably in the hominid line for certain.

Australopithecus anamensis followed, some million years ago. Soon after, the hominid line branched.

Hominid relationships  Hominids generally evolved in two directions. One, a “robust” line that became extinct about 1 million years ago. The other, is the “gracile” line continuing down to modern Homo sapiens.