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Unit Chapters
Genomics
Proteins & Proteomics
Evolution & Phylogenetics
Microbial Diversity
Emerging Infectious Diseases
HIV & AIDS
Genetics of Development
Cell Biology & Cancer
Human Evolution
Introduction
New Fossils
What Does DNA Tell Us About Our Position Among the Apes?
Variation Within and Among Human Populations
Out of Africa?
Neaderthals in Our Gene Pool?
Human Genetic Variation and Disease
Malaria, Sickle Cell Anemia, and Balancing Selection
Resistance to HIV
The Genetics of Asthma, a Complex Disease
Our History, Our Future
Neurobiology
Biology of Sex & Gender
Biodiversity
Genetically Modified Organisms
New Fossils

During the 1990s archaeologists unearthed dozens of new fossil hominids. These have been particularly useful for illuminating the changes that took place as the human lineage split from the chimp lineage. One important find was in Ethiopia by Tim White (University of California-Berkeley), Gen Suwa (University of Tokyo), and others who found a fossil, which they determined to be 4.4 million years old. The fossil, Ardipithecus ramidus, probably represents a transitional form with respect to the evolution of bipedalism: while it may have been able to walk upright, it had a different posture than we do. It probably spent some time upright and some time walking like a chimp, on its knuckles. In other respects, it looked much like chimp, except for subtle differences in teeth and skull. The first, clearly bipedal hominids - Australopithecus anamensis and Australopithecus afarensis - appeared about 4.1 million years ago, shortly after A. ramidus.

Figure 2. Human fossil "bush"
Other fossil discoveries illustrate the bushiness of the human lineage. As seen in the illustration, as many as four different apparent species often lived at the same time (Fig. 2). While there was a general trend toward increased brain size with time, species with considerably different brain sizes lived simultaneously. Questions remain about how different species replaced previous ones. Was it through warfare? Was it that the replacing species were better competitors? Perhaps it was simply a random event. We don't really know.

Despite the inferences we can draw from these new fossil findings, the fossil record still has limitations; it is incomplete. How can one determine whether different fossils belong to the same species? Species determinations are based on the ability, or the perceived ability, of different groups to interbreed. In cases where it is infeasible or immoral to do experiments crossing the two groups, one can infer the capacity for the groups to interbreed based on genetic data. Yet, with few exceptions, scientists cannot extract DNA evidence from fossils; only morphological characters are available. How then can one make the inferences about the capacity to interbreed? For instance, sexual dimorphism may lead one to classify males and females of the same population as separate species.

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