Evidence on Hominine Evolution

Evidence on Hominine Evolution

Lecture 19

It gives you some sense of the extreme excitement of nding 40% of a skeleton. Paleontologists are used to meager rations. They can get very excited if they can nd one tooth or one knucklebone.

Like many parts of this course, the modern story of human evolution is very recent. Even 50 years ago, we had far less information than we have now. Before completing the story of human evolution, we need to survey the evidence on which it is based. The evidence falls into three main categories: archaeological evidence, evidence based on the study of modern primates, and evidence based on genetic comparisons between modern species of primates, including ourselves. The most important evidence comes from surviving remains of our ancestors and the objects they left behind. Such evidence can tell us much about the physiology of our ancestors and their diets and lifeways. Some of the most exciting hominine remains have come from the African Rift Valley, the tectonic tear running from Mozambique through Tanzania and Kenya to Ethiopia.

In 1974, in Hadar, Ethiopia, Don Johanson and his colleagues found almost 40% of the remains of a hominine individual about three and one-half feet tall. This is one of the most complete hominine skeletons ever found. Johanson christened the remains “Lucy,” because his team had been listening to the Beatles song “Lucy in the Sky with Diamonds.”

Lucy illustrates well what skeletal remains can tell a skillful team of archaeologists. Radiometric dating of nearby materials determined that Lucy lived about 3.2 million years ago. Study of the pelvis showed that Lucy was female. Study of the teeth and other anatomical features suggested Lucy belonged to the genus of australopithecines (genus is the next taxonomic level above species) and that she died in her twenties.

Study of the pelvis and the base of the skull proved that she was bipedal. In quadrupedal species, the spine enters the skull from behind, not from below.

Study of the pelvis and the base of the skull proved that she was bipedal. In quadrupedal species, the spine enters the skull from behind, not from below. Yet her brain (at about 450 cc) was only slightly larger than that of a chimp (chimp brains average 350 cc). Clearly, bipedalism evolved before large brains. In 1978, Mary Leakey discovered fossilized footprints left by three australopithecines who had walked through still-warm lava, in Laetoli, Tanzania (also in the rift valley), about 3.6 million years ago. They con rmed that australopithecines were bipedal. Skeletons can tell us much more. For example, paleo-dentists can tell whether a tooth was used to eat plants or meat, and knowledge of diets can tell us a lot about lifestyles.

Louis Leakey (1903–1972) and his family made Olduvai gorge, also on the rift valley, one of the most famous of all sites for human paleoanthropology. Like the South Dakota badlands, this is an area where geological processes break open the Earth’s crust for us, revealing large numbers of fossil remains. In 1964, Leakey’s son Jonathon found a 2.3-million-year-old skull that was about half the size of a human skull (about 600 cc). Despite the smallness of the skull, Louis Leakey announced that a new species had been found, and he classi ed it as Homo habilis, placing it within the same genus as us.

Leakey insisted on classifying these remains within the genus Homo because habilis made tools. Their tools are known as “Oldowan,” after the Olduvai Gorge. Oldowan stone tools were made from pebbles of quartz, int, or even obsidian that were struck together to remove akes that could be used as cutting edges. The leftover cores may have been used as crude hammers. Leakey was impressed because these tools, though unsophisticated by later standards, suggested the sort of technological creativity he expected from humans. Modern attempts to manufacture Oldowan tools suggest considerable skill was needed for their manufacture. Microscopic study of stone tools can suggest what materials they were used on, and this can tell us something about diets and lifeways. For example, Homo habilis seem to have had an omnivorous diet dominated by plant foods such as leaves and fruits. Microscopic studies of cut marks and scratches on animal bones at hominine sites can determine whether hominines hunted for themselves or scavenged animals killed by others. The cut marks in icted by the stone tools of habilis often lie over the teeth marks of carnivores, which suggests that they generally scavenged from animals already killed by other carnivores.

A second vital type of evidence for understanding hominine evolution comes from studies of closely related species that are still alive today. Richard Leakey’s former students, Jane Goodall and Dian Fossey, pioneered the study of apes in the wild. They showed that the great apes have complex and clearly de ned social relationships that differ from species to species: For example, males compete for dominance. They also showed that humans are not the only great apes to use tools. For example, chimps often strip leaves from sticks to sh out termites from termite mounds. Though no apes make tools as sophisticated as those of habilis, this nding undermined Leakey’s claim that habilis was the rst great ape species to use tools.

The most recent technique for studying human evolution uses genetic evidence from living species. The evolution of genetic dating techniques counts as one part of the “chronometric revolution” described in Lecture Four. Genetic dating was pioneered by Alan Wilson and Vincent Sarich in the 1960s. How does it work? Many genes are not expressed in the physical body, so they do not all affect a species’ “ tness.” Such genes can therefore change randomly, so we can use statistical methods to estimate how much random genetic change there has been between two different species. By calibrating these differences against other evidence (such as the knowledge that mammal species diverged rapidly after the Cretaceous extinctions of 65 million years ago), we can estimate when two species may have shared a common ancestor. When they rst proposed the idea, Wilson and Sarich met with great skepticism, but since then, genetic dating has become a fundamental tool for the study of evolution in general. Genetic dating techniques have revolutionized our understanding of human paleontology by showing that the DNA of humans and chimps differ by little more than 1%. This suggests that the two species had a common ancestor about 7 million years ago, rather than 15–20 million years ago as suggested by earlier studies of skeletal remains. This date establishes a clear time frame for the history of human evolution. Such techniques are particularly important because so few fossil remains survive from this era. We’ve seen some of the evidence used to construct the story of human evolution, and now we can complete the story. The next lecture asks: What was it that made our species so different?

Essential Reading

Supplementary Reading

Questions to Consider

Christian, Maps of Time, chap. 6. Fagan, People of the Earth, chap. 2

Johanson and Edey, Lucy. Jones, The Cambridge Encyclopedia of Human Evolution. Lewin, Human Evolution.

1. What are the most important forms of evidence used to reconstruct the evolution of our species?

2. How have techniques of genetic dating transformed our understanding of hominine and human evolution?