The African Eve Hypothesis

Attempts to sort out human evolution from skulls and other bones led to two alternative schemes. The multi-regional model proposes that Homo erectus evolved gradually into Homo sapiens simultaneously throughout Africa, Asia and Europe. The Noah's Ark model proposes that most branches of the human family became extinct and were replaced, relatively recently, by descendants from only one local sub-group (Fig. 20.23). Although anthropologists take both theories seriously, few geneticists regard the multi-regional model as plausible. This model implies continuous genetic exchange between widespread and relatively isolated tribes over a long period of prehistory. Not surprisingly, recent molecular analysis has tended to support the Noah's Ark model.

Although mitochondria evolve fast, the overall variation among people of different races is surprisingly small. Calculations based on the observed divergence and the estimated rates suggest that our common ancestor lived in Africa between 100,000 and 200,000 years ago. Since mitochondria are inherited maternally, this ancestor has been named "African Eve". This African origin is supported by the deeper "genetic roots" of modern-day African populations. In other words, different sub-groups of Africans branched off from each other before the other races branched off from the Africans as a whole (Fig. 20.24).

The ancestors of today's Europeans split off from their Euro-Asian forebears and wandered into Europe via the Middle East around 40,000 to 50,000 years ago (Fig. 20.25). American Indians appear to derive from two major migrations originating from mainland Asian populations. The earlier Paleo-Indians (around 30,000 years ago) populated the whole American continent, while the more recent migration (less than 10,000 years ago) produced the Na-Dene peoples who are mostly North American Indians.

Besides using mitochondrial DNA, sequences of microsatellite regions of the chromosomes have been compared among the different races. The phylogenetic results are very similar. They also give a primary African—non-African split, and if anything, they suggest an even more recent date for the common ancestor, nearer 100,000 years ago.

But what about Adam, or "Y-guy" as he is sometimes called by molecular biologists? The shorter human Y chromosome does not recombine with its longer partner, the X chromosome over most of its length. This allows us to follow the male lineage without complications due to recombination. For example, the ZFY gene on the Y chromosome is handed on from father to son and is involved in sperm maturation. The sequence data for ZFY suggest a split between humans and chimps about 5 million years ago and a common male ancestor for modern mankind about 250,000 years ago. However, recent data from a much larger number of genetic markers on the Y chro-

African Eve Hypothetical female human ancestor thought to have lived in Africa around 100,000-200,000 years ago Y-guy Hypothetical male human ancestor thought to have lived in Africa around 100,000-200,000 years ago

Mitochondrial sequence analysis suggests all modern humans are derived from a small group of ancestors who lived in Africa around 100,000 years ago.

Europe Africa Asia

Europe Africa Asia

Europe Africa Asia

Modern sapiens

Archaic sapiens

Homo erectus

Europe Africa Asia

Multiregional model

NOAH'S ark MODEL

FIGURE 20.23 Multi-Regional and Noah's Ark Models of Human Evolution

The multiregional model of human evolution (left) suggests that Homo sapiens developed from multiple interactions between several ancestral lines. The early Homo erectus ancestor branched and migrated from Africa to Asia and Europe. Traits developing in each branch were transmitted to the other branches implying genetic exchanges between the three branches, even though many thousands of miles separated the early ancestral groups. The Noah's Ark model (right) seems more plausible based on genetic analysis. The model suggests that modern Homo sapiens developed from one ancestral group in Africa. Other branches of archaic sapiens did develop and inhabited different regions in Europe and Asia for a while before dying out. The modern sapiens branch has then evolved into several branches from a relatively recent African ancestor.

FIGURE 20.24 African Eve Hypothesis l—DNA

This phylogenetic relationship was deduced by comparing mitochondrial DNA sequences from living humans. Numbers shown are estimated years before the present (BP). According to the African Eve theory, early humans developed in Africa about 150,000 years BP and diverged into many different tribal groups, most of which remained in Africa. The European and Asian races are derived from those relatively few groups of African ancestors who emigrated into Eurasia via the Middle East.

American

American

Y chromosome sequences confirm the recent African origin of humans.

mosome dates Y-guy to somewhat less than 100,000 years ago. Recent analyses of clusters of mutations on the Y chromosome in are incompatible with the multi-regional model and confirm the recent African origin of modern humans.

A final sad note concerns Neanderthal Man. Although Neanderthal Man survived to live alongside the modern races of Homo sapiens in Europe and the Middle East until relatively recently (approximately 30,000 years ago), sequence analysis suggests that the Neanderthals came to a dead end. Comparison of DNA sequences suggests

Olduvai Gorge modern Home sapiens probably evolved near here

FIGURE 20.25 African Eve Hypothesis H—Migrations

Olduvai Gorge modern Home sapiens probably evolved near here

FIGURE 20.25 African Eve Hypothesis H—Migrations

The divergence of the African ancestor into the modern African, European and Asian races included migration into different parts of the world. Scientists believe that modern Homo sapiens evolved in eastern Africa, around the Olduvai Gorge. Descendents of these early ancestors migrated to Europe and Asia as well as other areas in Africa. Descendents of some Asian groups crossed the Bering Strait to inhabit the American continent. Once isolated, these various groups evolved independently.

Small stretches of DNA sequence have been rescued from extinct life forms.

that the Neanderthals did not interbreed with modern Man or contribute significantly to the present-day human gene pool.

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