Human Ancestors: The Search Continues
Human Ancestors: The Search Continues
The search for human ancestors and origins made great strides in the second half of the twentieth century. During the nineteenth and early twentieth century few actual fossils were known, it was thought that there were only a few types of archaic (early) people, and the age of those fossils was in dispute. Today, sophisticated dating techniques and DNA analysis have been brought to bear upon the question of age. Also, far more fossil material has been found showing that the Neanderthals and Cro-Magnons were not alone. The evidence proves that there were many early hominids (human-like creatures). Paleoanthropologists (those scientists who study ancient humans) agree that the cradle of mankind was not Central Asia, as was once thought, but Africa—an idea that suggests we are all of African descent regardless of our skin color. While more is known of our ancestors than ever before, questions and controversies still rage over just how humans evolved to the stage we are at today.
Evidence that the first human ancestors originated in Africa was discovered by Raymond Dart (1893-1988) as early as 1924, though few at that time believed such proof was conclusive. In fact, Charles Darwin (1809-1882), in his Descent of Man (1873), had hypothesized that Africa would prove the cradle of man, but he was ignored. Dart was a young professor of anatomy at the University of Witwatersrand in Johannesburg, South Africa. Some interesting primate fossils had been discovered by workers in a limestone quarry in the nearby region of Taung, and Dart arranged to have them sent to him. In a box of these fossils Dart noticed the broken remains of a tiny skull that was neither a baboon nor other known primate. Over time he pieced the fragments together to reveal the skull of a child. He dubbed the diminutive relic Australopithecus africanus (the South African Ape) and decided it was an intermediate stage between anthropoids (monkeys) and man.
Dart published a description of the fossil shortly thereafter, but few scientists seemed interested in what Dart thought to be the earliest known human ancestor ever found. Those who were interested found it so because it suggested that the primates had ventured much further south during their evolution than previously thought. They did not think it a human ancestor, however. Dart argued that it was a human ancestor because the Foramen Magnum—the hole in the skull that the spinal cord passes through—was in the bottom like a human, not in the back like a primate. Therefore, he reasoned, the creature must have walked upright on two feet. Analysis of the fossil specimen proved problematic because it was an infant (organisms often change their morphology, or physical structure, as they grow to adults) and there were no other fossil parts to go with it. Also, the skull had both primate and human characteristics. As such "Taung Child," as Dart called it, was dispensed by the scientific world to a strange limbo of existence.
Convincing evidence that Africa was the cradle of human origins was discovered in the 1950s and 1960s by the husband and wife team of Louis and Mary Leakey. Louis Leakey (1903-1972) was born in Kenya and was raised with the children of the local Kikuyu tribe. He studied anthropology at Cambridge University, England, and became obsessed with discovering the oldest humans, and convinced that those humans would be found in Africa. Mary Leakey (1913-1996) was Louis's second wife and shared his fascination with human ancestry. They made their discoveries in a wasteland area of Tanzania known as Olduvai Gorge. The area is excellent for fossil hunting because many layers of rock, or strata, are exposed, enabling fossils found at that location to be dated. The Leakey's originally came to Olduvai Gorge in the early 1930s. They discovered numerous broken stones that they believed were actually the oldest known tools made by humans. The problem was that they could find no human fossil remains to go with those tools. By the 1950s the Leakeys were able to return to the area to do a more systematic search.
One day in 1959, while Louis lay in bed with a fever, Mary went prospecting and discovered a set of fossil hominid teeth set in a jaw. Electrified by the find, the team scoured the area for more and eventually pieced together an entire skull. They also discovered more stone tools. Chemical dating later showed the skull specimen to be 1.75 million years old. Dubbed Zinjanthropus boisei (East African Man) by Mary Leakey, the skull represented a creature directly ancestral to modern humans.
Louis Leakey had never thought Raymond Dart's Australopithecus to be ancestral to man, but in fact Zinjanthropus was an australopithecine. Leakey saw his creature as more modern than Dart's and directly related to humans. He refused to accept Zinj, as Leakey called it, as anything other than a human ancestor. This caused a debate over just what group the skull fell into. Eventually, Leakey came around to the fact that his Zinj was an Australopithecus.
With their success at Olduvai Gorge the Leakey's became famous and thus had less trouble getting the money necessary to continue their work. Louis became curator of the Coryndon Museum, while Mary was able to take over the day to day control of the dig site. Their son Jonathan assisted his mother in discovering a different type of hominid that was clearly different from Zinj. Soon dubbed Homo Erectus (the upright man), this creature stood fully on two legs like a modern human. Louis did return to Olduvai when he could. In 1962 he found hominid parts that were later ascribed to the earliest known human toolmaker, which Leakey called Homo habilus (the handy man). The term Homo designates a creature as being fully in the same family as modern humans.
However, they discovered more than just fossils and tools. In May 1970 Mary Leakey uncovered a series of fossil footprints near Laetoli, Tanzania, not far from Olduvai Girge. The track-way was especially significant because, while fossilized bones are representations of a dead creature, trackways are representative of a creature that is alive and moving about. The footprints, made in volcanic ash, show what some argue was a group, possibly a family, of archaic hominids—a male, a female, and a child—walking together, perhaps holding hands. They were walking upright 3.75 million years ago.
The entire Leakey family took part in the discovery of human ancestry. Their youngest son, Richard Leakey (1944- ), also made important contributions to the study of paleoanthropology. Richard Leakey made his discoveries around Lake Turkana, Kenya. This fossilladen area is known as the Koobi Fora. In 1972 his team (particularly his field assistant Bernard Ngeneo) discovered several hundred fragments that were carefully pieced together to create a skull designated 1470. It was a remarkable three million years old and had a relatively large brain. Neither Richard Leakey nor his colleagues could agree whether 1470 was an Australopithecus or Homo. Skull 1470 may have represented a different hominid line, suggesting that several groups had lived in the same area at the same time—a possibility not generally accepted within scientific circles of the day. If there were more than one type of hominid in Africa at the same time, the entire history of human ancestry, and how scientists viewed that ancestry, would have to be reconsidered.
The next major discovery in the search for human ancestors was made in Hadar, Ethiopia, in 1974. A team led by American paleoanthropologist Donald Johanson discovered the most complete Australopithecus skeleton ever found. Nicknamed "Lucy"—after the Beatles's song "Lucy in the Sky With Diamonds"—the creature was a true biped, weighing about 60 pounds and standing about three and a half feet tall. Fossils can not be directly dated. Because their approximate age is established by dating the matrix, or surrounding rock, in which they are found. In this manner Lucy was found to be roughly 3.18 million years old.
After Lucy was uncovered, Johanson's team found the remains of over a dozen individuals. Though they seemed similar to Lucy, they were older and physically larger. As they were found together, they were dubbed the "First Family." It was later realized that these creatures did not die as a group, but were washed into the location by drowning at different times. Johanson made his discoveries public in 1978. He argued that the group of creatures Lucy was a part of (not her as an individual) represented the earliest human ancestors. In fact, he argued that humans as a group find their point of origin in these creatures. With that he created a new hominid category—something only rarely done and with much serious consideration and evidentiary support—called Australopithecus aferensis. This made the human line almost four million years old.
Fossils represent facts that are generally firm. Interpretation of those facts is another question entirely. It is in the interpretation of these fossils that problems arise. The central question is one of relationship: how does one group of fossils relate to another in time and space? Just because one genus of human fossils is older than another does not necessarily mean that one gave rise to the other. The old linear model of human evolution asserted that one group appeared and evolved into another (with the old group dying off), which then evolved into another, and so on to modern humans. It is a simple straightforward view: One group at a time inhabiting the world to be pushed out by the next group. Because of the finds discussed above, this simplistic model is no longer accepted.
It is clear now that different groups of archaic humans inhabited the earth along side other archaic humans, as well as advanced primates. Groups overlapped each other. Some groups gave rise to new variations, while some groups died off as evolutionary dead ends. Just what that progression was remains a major question. There are several theories that are put forward to explain the progression of human evolution. One popular idea is the Out of Africa theory. This theory states that a group of advanced hominids walked out of Africa several million years ago. As they spread out they evolved into other forms until the appearance of modern Homo sapiens (the smart man) about 150,000 years ago. There is also the Out of Africa Two theory, which states that one group did leave Africa to evolve into more complex forms, but was followed by a second wave that left Africa later and competed with those hominids already out in the world.
Another version of evolution is the single species theory, which claims that all humans, whether extinct or extant, are of one species, but variations on a general theme. There is also the Eve hypothesis, which states that a tiny isolated group of early hominids, probably in Africa, were the original ancestral stock of all modern humans. A more controversial view is the Multi-Regional hypothesis, which argues that modern humans appeared in several different locations at different times, completely independently of one another, and gave rise to the different modern human groups. In contrast, there is the Diversity hypothesis, which contends that there were many different human species living side by side at one time, but that for one reason or another only one species survived. That surviving species gave rise to all modern humans, making us all the same species regardless of superficial differences.
At this point there is no consensus among scientists about the relationship of one group of archaic humans and hominids to another, or their relationship to modern humans. The search for answers to these puzzling questions continues, as does the debate about what it all means.
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Tattersall, Ian. The Fossil Trail: How We Know What We Think We Know about Human Evolution. New York: Oxford University Press, 1995.