Early Hominin Evolution: Discovery of Early Hominids

The immediate ancestors of humans were members of the genus Australopithecus . The australopithecines (or australopiths) were intermediate between apes and people. However, both australopithecines and humans are biologically similar enough to be classified as members of the same biological tribe--the Hominini . All people, past and present, along with the australopithecines are hominins . We share in common not only the fact that we evolved from the same ape ancestors in Africa but that both genera are habitually bipedal , or two-footed, upright walkers. By comparison, chimpanzees, bonobos, and gorillas are primarily quadrupedal , or four-footed.

Over the last decade, there have been a number of important fossil discoveries in Africa of what may be very early transitional hominins, or proto-hominins. These creatures lived about the time of the divergence from our common hominid ancestor with chimpanzees and bonobos, during the late Miocene and early Pliocene Epochs. The fossils have been tentatively classified as members of three distinct genera--Sahelanthropus , Orrorin , and Ardipithecus . However, all of their fossil remains are still fragmentary and far from being complete. Sahelanthropus was the earliest, dating 7-6 million years ago. Orrorin lived about 6 million years ago, while Ardipithecus remains have been dated to 5.8-4.4 million years ago. At present, the vote is still out as to whether any of these three primates were in fact true hominins and if they were our ancestors. The classification of Sahelanthropus has been the most in question.

The earliest australopithecines very likely did not evolve until 5 million years ago or shortly thereafter (during the beginning of the Pliocene Epoch) in East Africa. The primate fossil record for this crucial early transitional period leading to australopithecines is scanty and somewhat confusing at present. However, by about 4.2 million years ago, unquestionable australopithecines were present. By 3 million years ago, they were common in both East and South Africa. Some have been found dating to this period in North Central Africa also. As the australopithecines were evolving, African forests were progressively shrinking and grasslands, or savannas , were advancing. In this sort of environment, bipedalism would very likely have been an advantage.

By 2.5 million years ago, there were at least 2 evolutionary lines of hominins descended from the early australopithecines. One line apparently was adapted primarily to lake margin grassland environments and had an omnivorous diet that increasingly included meat. Among them were our early human ancestors who started to make stone tools by this time. The other line seems to have lived more in mixed grassland and woodland environments, like the earlier australopithecines, and was primarily vegetarian. This second, more conservative line of early hominins died out by 1 million years ago or shortly before then. It is likely that all of the early hominins, including humans, supplemented their diets with protein and fat rich termites and ants just as some chimpanzees do today.

In an 1871 publication, Charles Darwin speculated that fossils of the earliest humans and their primate ancestors ultimately would be found somewhere in Africa. He based this on the fact that the natural range of our nearest living relatives, chimpanzees and gorillas, is limited to Africa. He concluded that we ultimately must have shared a common ancestor with those apes in Africa. This view was mostly rejected by the scientific world of the time. Before the 1920's, knowledge of our fossil ancestors only went back to the Neandertals in Europe and some presumably earlier human-like forms from Java, in Southeast Asia. Few researchers were willing to estimate the time period of the earliest hominins at much more than 100,000 years, and there was no inkling of anything older from Africa. In addition, there was a bias among the predominantly European paleoanthropologists against accepting early Africans as the ancestors of all humanity.

In 1924, Raymond Dart, an Australian anatomy professor at the University of Witerwatersrand in Johannesburg, South Africa, obtained a fossil skull that had been blasted out of a nearby limestone quarry at Taung . It took him 73 days to chisel the skull free from its surrounding stone matrix and ultimately 4 years of spare time to free the jaw and the fossilized brain. However, long before then, Dart recognized the importance of this find. In 1925 he named it Australopithecus africanus (literally "southern ape from Africa"). Because of its small size, he called it the "Taung baby." In fact, it was a child of 3-4 years old. Despite its relatively small brain, he concluded that this species was intermediate between apes and humans. He based this mainly on the shape and position of the base of the brain. It indicated that the foramen magnum , or hole in the skull through which the spinal cord passes, pointed downward and was nearly at the central balance point of the skull. This meant that the Taung child must have been bipedal. In addition, the canine teeth were relatively short. In both of these traits, the Taung child was much more like a human than an ape. Most paleoanthropologists in the 1920's rejected Dart's claims that Australopithecus africanus was intermediate between apes and humans in favor of the view that it was just an ape. Dart's claims were not widely accepted until the late 1940's.

Following Dart's discovery, several other caves were investigated in South Africa. Most of the work was done by Robert Broom from 1936 through the 1940's. Broom was a medical doctor and an enthusiastic amateur paleontologist from Scotland. In 1903, he was appointed professor of geology at the University of Stellenbosch in South Africa and became internationally respected for his studies of early mammal-like reptiles. His insistence on the correctness of the theory of evolution led to his dismissal from this conservative religious university in 1910. Consequently, he returned to being a medical doctor in a relatively remote part of South Africa but continued paleontological research in his spare time. In 1934, at the age of 68, he retired from his medical practice and joined the staff of the Transvaal Museum in Pretoria as a paleoanthropologist. The rest of his life was spent searching for early hominin fossils.

Robert Broom's most important discoveries were made in the Sterkfontein valley of South Africa. It was there in 1936 that he found the first known adult Australopithecus africanus while excavating in Sterkfontein cave. In 1938, he discovered more fossil remains of africanus and other early hominins in Kromdraai cave. Some of these fossils were larger boned and more muscular with powerful jaws. Broom named them Paranthropus robustus (Paranthropus means "parallel to man"). Significantly, these robust hominins also differed in having a sagittal crest , or ridge of bone extending from front to back, along the midline of the top of the skull. A sagittal crest serves as an anchor attachment for exceptionally large, strong jaw muscles. This skeletal feature is also present in large apes but not in africanus or humans.

NOTE: Most paleoanthropologists discarded the name Paranthropus robustus by the 1960's in favor of Australopithecus robustus. However, some researchers in recent years have returned to the original genus designation. Paranthropus will be referred to as Australopithecus here.

In 1948, Robert Broom found more Australopithecus robustus fossils at Swartkrans cave in South Africa. Following that excavation, he dedicated the rest of his life to writing everything known about all of the early hominins. He completed this compendium work in 1951. He was 85 years old and ill. As he finally finished his writing, he reportedly said "now it is done and so am I". He died a few minutes later.

Between 1965 and 1983, Swartkrans cave was carefully reinvestigated by C. K. Brain using more thorough field and laboratory techniques than had been used by Robert Broom a generation earlier. Many thousands of bone fragments, including the remains of 130 individual hominins, were recovered by Brain. These bones were from both species of early hominins identified before as well as members of our genus, Homo. Because many of the bones had chewing marks and at least one of the skulls had peculiar depressions reminiscent of punctures made by the canine teeth of a leopard, Brain hypothesized that some of the Swartkrans hominins had been eaten by these big cats. The early hominin fossil-bearing strata in the cave also contained 195 stones that were from another location distant from the cave. Brain believed that 30 of them may have been used as tools or weapons. In any case, the presence of these stones suggests that not all of the early hominins in the cave were there as a result of being the victims of carnivores.Unfortunately, most of the South African sites where early hominin fossils have been found are not easily dated because they lack association with volcanic deposits that would readily allow radiometric dating. That is not the case with most of the early hominin sites in East Africa.

The oldest fossil hominins have been recovered from sites in East Africa, especially in the Great Rift Valley. One of the most important sites there is Olduvai Gorge . It is an approximately 30 mile (48 km.) long, eroded canyon complex cutting into the Serengeti Plain in Northern Tanzania. It is only about 295 feet (90 m.) deep, but its neatly stratified layers of dirt and rock interspersed with easily datable volcanic ash and lava layers cover the last 2.1 million years of geological and evolutionary history. The remains of many australopithecines and early humans have been found at Olduvai. When these ancient hominins lived there, it was a grassland that probably had abundant food sources.

Early hominin fossils from Olduvai Gorge are known mostly as a result of the many expeditions of Louis and Mary Leakey . Louis began searching there in 1931, and his second wife Mary joined him in 1935. However, it was not until 1959 that they found their first early hominin fossil. Louis gave it a new genus and species designation, Zinjanthropus boisei (literally "East African man"). Subsequently, it was recognized to be only a super robust species of australopithecines. It is now generally referred to as Australopithecus boisei . Using the then new potassium-argon dating method, the fossil was determined to be 1.75 � .25 million years old. This was a startlingly early date when it was made public over four decades ago. Louis Leakey and Zinjanthropus instantly became international media stars, and both of their pictures were on the front page of newspapers around the world. Louis was also the focus of several television documentary programs. In the years after his death in 1972, Mary became well known as a paleoanthropologist in her own right.

In 1974, a team of paleoanthropologists, under the direction of an American, Donald Johanson, found an even more ancient species of australopithecine at the Hadar site in the Afar Desert region of Northern Ethiopia. It was a 40% complete skeleton of an adult female who they named Lucy. She had been only 3 feet 3 inches (1 m.) tall with a slender body weighing only about 60 pounds (27 kg.). She lived 3.2-3.18 million years ago. Johanson concluded that Lucy was from a different species than had been previously discovered. He classified her as an Australopithecus afarensis (named for the Afar region). Many other specimens of this species and later ones were found in Ethiopia since 1974, but none is as complete as Lucy.

Finding Lucy--how she was discovered
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Becoming a Fossil--how Lucy was preserved
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About 30 miles south of Olduvai Gorge in Northern Tanzania is the Laetoli site. It was investigated in the late 1930's by Louis and Mary Leakey, but no fossil hominins were found at that time. Mary Leakey returned to Laetoli with Tim White, an American paleoanthropologist in 1978. They found bones of what were likely Australopithecus afarensis dating in the range 3.7-3.5 million years ago. They also found 59 footprints of bipedal hominins (presumably afarensis) in a now hardened volcanic ash layer. These individuals walked in two close parallel tracks across volcanic dust at least 3.5 million years ago. The footprints look almost like those of modern humans. They are narrow with a low arch, and they clearly show that the big toe was in line with the others. These are all traits of humans but not of apes. Based on the characteristics of the footprints, Mary Leakey concluded that their makers were adults who were 4 feet 9 inches and 4 feet 1 inch tall and that they walked parallel to each other in a strolling fashion with relatively short strides. In addition, there are a child's footprints within those of the larger adult.

Laetoli Footprints--how they were formed and preserved
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In northern Kenya and southern Ethiopia, there is an arid region around Lake Turkana in the Great Rift Valley that has exposed geological deposits dating to at least 4.3 million years ago. Richard Leakey, the son of Mary and Louis Leakey, began looking for hominin fossils there in the late 1960's. During the 1970's, his team of field researchers from the National Museum of Kenya made a number of important finds, including fossils of early humans who will be described in the next tutorial of this series.

While working on the western side of Lake Turkana in 1985, an American paleoanthropologist named Alan Walker made an important discovery. This was a nearly complete robust australopithecine skull with an unusually large sagittal crest and some features reminiscent of the more ancient Australopithecus anamensis (described below). Manganese in the soil deposit where it was located stained it black. As a result, this unusual fossil has become known as the "black skull." It has been classified as Australopithecus aethiopicus (named after Ethiopia). Since it dates to 2.5 million years ago, it is a prime candidate for being the earliest robust australopithecine species.

In 1995, Meave Leakey, the wife of Richard Leakey, began discovering bones of a very early australopithecine species at several sites southwest of Lake Turkana. She named it Australopithecus anamensis ("anam" is "lake" in the Turkana language). The dentition of this hominin seems to be transitional between apes and later australopithecines. This fits with the 4.2-3.9 million year dates for the volcanic ash associated with the anamensis fossils. The shapes of the arm and leg bones of this species indicate that it was bipedal.

In 1996, Berhane Asfaw, an Ethiopian researcher, and Tim White found a 2.5 million year old hominin fossil in the Middle Awash Valley of Ethiopia that remains problematical. The skull characteristics are similar in some ways to Australopithecus afarensis, but it lived several hundred thousand years after that species had presumably ended. Asfaw named his new hominin, Australopithecus garhi ("garhi" is "surprise" in one of the languages of the Afar Desert region). Whether or not garhi was a late afarensis, a variant of africanus, or a distinct species is not clear. Associated with the remains of garhi were animal bones with what appear to be cut marks made by simple stone tools. If they were using such tools, this is remarkable because only early humans have heretofore been associated with stone tool making.

In 2001, Meave Leakey announced the discovery of a 3.5-3.2 million year old hominin skull from the west side of Lake Turkana. She suggested that this fossil may displace Lucy (Australopithecus afarensis) as the progenitor of humans. Meave named it Kenyanthropus platyops ("flat-faced man of Kenya"). This hominin lived during the same time period as Lucy. However, it had a comparatively large, flat face and smaller teeth. The latter characteristic suggests that Kenyanthropus regularly ate softer foods than did Lucy. However, it is not yet clear where this new discovery fits within our evolution. Some paleoanthropologists have suggested that it is only a variant of Australopithecus afarensis.

In 2006, Tim White of the University of California, Berkeley, announced the discovery of bones from at least 8 Australopithecus anamensis individuals dating to 4.1 million years ago in what had been a woodland environment in the Awash Valley of Ethiopia. White believes that anatomically they are direct descendents of Ardipithecus ramidus and direct ancestors of Australopithecus afarensis. In other words, he asserts that there was a single evolutionary line of the earliest hominins without branching out into other species and that this evolutionary development occurred in East Africa in an area that included at least the Great Rift Valley in Kenya and Ethiopia.

NOTE: Our understanding of early hominins was led astray at the beginning of the 20th century as a result of the discovery by Charles Dawson in 1912 of a fossil skull in England that became known as the Piltdown man. It had a large brain case similar to modern humans but an ape-like jaw. This fit with the popular but incorrect assumption that our early ancestors would have ape-like bodies and human-like brains. The discovery of australopithecines in South Africa beginning in 1924 showed that the early hominins were actually just the reverse―they had almost human-like bodies below the neck but brains that were very little changed in size from those of apes. It was not until the early 1950's that the Piltdown man skull was exposed for what it really was, a clever fraud. This realization came as a result of close examination by independent researchers and fluorine analysis dating.


	Australopithecus africanus	Paranthropus robustus (Australopithecus robustus) (see note below)



	Leopard canines fit punctures in hominin skull from Swartkrans


Mary and Louis Leakey with the "Zinjanthropus boisei" palate and a modern human skull in 1959


Raymond Dart (1893-1989)

MAJOR EARLY HOMININ SITES


	Zinjanthropus boisei (Australopithecus boisei)



"Black Skull" (Australopithecus aethiopicus)