THE FIRST HUMANS
Because of its association with tools, the earliest humans are called Homo habilis, meaning _handy man._ Partial skeletons indicate that H. habilis was small in stature, with arms longer than legs and a skeleton much like an ape. There is great diversity among early Homo fossils, and anthropologists are increasingly taking the view that human evolution has not been a simple linear process, as has been traditionally believed. Rather, the human evolutionary tree appears to be a "bush" exhibiting diverse branches.

DISCOVERY OF HOMO ERECTUS
The first fossils of a larger, bigger brained species of Homo, H. erectus, were first discovered in Java by Eugene Dubois in 1891. Dubois_ fossil hominid had long, straight legs and was an excellent walker. The size of the skull suggested a very large brain, about 1000 cubic centimeters_much larger than any ape's. In the years since, about 40 individuals similar in character and age to his fossil have been found in Java, and even more near Peking (now Beijing), China. Java man and Peking man are now recognized as belonging to the same species_Homo erectus. At a height of 5 feet, H. erectus was much taller than H. habilis. Homo erectus had a large brain, about 1000 cubic centimeters, and walked erect. Its skull had prominent brow ridges and, like modern man, a rounded jaw. The shape of the skull interior suggests that H. erectus was able to talk.

HOMO MIGRATES OUT OF AFRICA
In 1976, a complete H. erectus skull discovered in East Africa was dated at 1.5 million years old, a million years older than the Java and Peking finds. The appearance of H. erectus in Africa 1.5 million years ago marked the beginning of the great human expansion. Far more successful than H. habilis, H. erectus quickly spread and became abundant in Africa and within a million years had migrated into Asia and Europe.

Homo erectus survived longer than any other species of human, for nearly a million years. These very adaptable humans only disappeared in Africa about 500,000 years ago, as modern humans were emerging. Interestingly, H. erectus survived much longer in Asia, until about 250,000 years ago.

OUT OF AFRICA_AGAIN?
The evolutionary journey to modern humans entered its final phase when modern humans, H. sapiens ("wise man"), first appeared in Africa about 600,000 years ago. Because the first fossil to be found, in 1856, was from the Neander Valley of Germany, early European humans were called Neanderthals (thal means "valley" in old German). Originating in Africa, the line which led to Neanderthals appears to have branched off from the ancestral line leading to modern humans as long as 500,000 years ago_mitochondrial DNA recovered from fossils is that different from living human mDNA. Compared with modern humans, European Neanderthals were short, stocky, and powerfully built. Their skulls were massive, with protruding faces and heavy, bony ridges over the brows. Their brains were larger than those of modern humans. They have been extinct for over 30,000 years.

The origin of today's human races is a much-debated point among scientists studying human evolution. Many biologists believe that H. sapiens evolved only once, in Africa, and then migrated to Europe and Asia. Acceptance of a unique African origin of H. sapiens (the so-called Out-of-Africa model) implies that modern-day Asian H. sapiens evolved from African ancestors; an opposing view (the so-called "Multiregional" model) argues that the human races independently evolved from H. erectus in different parts of the world. Investigators favoring the Multiregional model argue that each of the races of H. sapiens evolved in situ from H. erectus and that each adapted to a different environment. Others favoring the Out-of-Africa model believe that the same species would be unlikely to evolve more than once and argue that human races appeared after H. sapiens evolved from H. erectus in Africa.

THE MITOCHONDRIAL DNA CONTROVERSY
In 1987, scientists at Berkeley, California, studying mitochondrial DNA from humans all over the world made the startling claim that all human races originated from one H. sapiens ancestor in Africa. They chose mitochondrial DNA to study hominid evolution because the DNA within mitochondria is transmitted only by females. Female's eggs carry many mitochondria that become part of a new baby, while sperm contribute no mitochondria to the new baby (sperm carry their mitochondria wrapped around their tails and do not inject them into the egg during fertilization). For that reason, particular versions of a mitochondrial gene can be traced back through a family tree, from mother to grandmother to great-grandmother.

However, after much initial excitement about the "mitochondrial Eve," the result is no longer widely accepted. The computer program the Berkeley scientists used to reach their conclusion, called PAUP (for Phylogenetic Analysis Using Parsimony), gives different results depending upon how the data are entered! Thus, the exact human tree cannot be reliably traced using this approach.

However, the greatest number of different mitochondrial DNA sequences occur among modern Africans. Because DNA accumulates mutations over time, the oldest populations should show the greatest genetic diversity. This result is consistent with the hypothesis that humans have been living in Africa longer than in any other continent, and from there spread to all parts of the world, retracing the path taken by H. erectus half a million years before.

A CLEARER PICTURE FROM NUCLEAR DNA
A clearer analysis is possible using chromosomal DNA, some highly variable segments of which are far more variable than mitochondrial DNA, so providing more "markers" to compare. In 1996, a large team of geneticists from six countries reported the results of a comprehensive nuclear DNA study. The team analyzed, in human populations all over the world, two variable parts of the CD4 gene (the cell surface receptor on white blood cells that recognizes the AIDS-causing HIV virus) located on human chromosome 12. They examined 1,600 individuals in forty-two populations. A total of 24 different versions of the two segments were found. Fully 21 of them were present in human populations in Africa, while three were found in Europeans and only two in Asians and in Americans. This argues forcefully that chromosome 12 has existed in African humans far longer than in non-African humans. Together with fossils of early H. sapiens from Africa and Israel, these results strongly support an Out-of-Africa model of human origins. Thus, while still hotly debated, it appears likely that H. sapiens evolved once in Africa, then migrated out of Africa to Europe and Asia.