|The Out-Of-Africa Hypothesis, Mitochondrial Eve and Y-Chromosome Adam|
Evolution is the theory by which living creatures acquire and pass on randomly-acquired new traits from generation to generation, affecting the overall make-up of the population and even leading to the emergence of new species. This theory, first proposed by Charles Darwin, led to the generaly accepted scientific theory that humans evolved from earlier pre-human primates. During the early 20th century most scientists believed that humans evolved from separate groups of primates independently in Asia and Africa. In the last few decades scientific opinion has shifted and it is now generally believed that all humans can trace their ancestry back to a single man and a single woman, from Africa. Some Christians strongly oppose the theory of evolution as incompatible with Scripture, others take the early chapters of Genesis in a more poetic and metaphorical sense and so do not see evolution as contradicting the Bible.
The Out-of-Africa hypothesis is also known as the single-origin hypothesis (or Out-of-Africa model) and as stated above, is one of two accounts of the origin of anatomically modern humans, Homo sapiens. (The other theory is the multiregional hypothesis.)
Pre-modern (non-sapiens) hominids
Because of the scarcity of fossils and the discovery of important new finds every few years, researchers disagree about the details and sometimes even basic elements of human evolutionary history. While they have revised this history several times over the last decades, researchers currently agree that the oldest named species of the genus Homo, Homo habilis, evolved in Africa around two million years ago, and that members of the genus migrated "out of Africa" somewhat later. The descendants of these ancient migrants, which probably included Homo erectus, have become known through fossils uncovered far from Africa, such as those of "Peking man" and "Java man". The Homo neanderthalensis is also considered a descendant of early migrants.
According to the single-origin model, however, every species of the genus Homo but one, Homo sapiens, was driven extinct. This species had evolved in eastern Africa between 100,000 and 200,000 years ago and, some time afterwards, in a relatively recent exodus, began colonizing the rest of the world. According to the single-origin model, these more recent migrants did not interbreed with the scattered descendants of earlier exoduses. For this reason, the model is sometimes called the "replacement scenario". In support of it, advocates have drawn from both fossil and DNA evidence, in particular from mitochondrial and Y-chromosome DNA sequences. Research on the X chromosome claims to be the first genetic evidence against Out of Africa.
Single exodus from Africa?
Assuming only relatively recent migrants from Africa gave rise to today's non-African humans, was there more than one migration that left descendants? (for example, one each via the north and south ends of the Red Sea)
- Stephen Oppenheimer is one proponent of a single exodus
- John Hawks in Playing games with dates criticizes current presentations of the idea
The opponents of a single origin argue that interbreeding indeed occurred, and that the characteristics of modern humans, including those that have been and still are perceived by some to distinguish races, could only be the result of genetic contributions from several earlier lineages that evolved semi-independently in different parts of the world. This is the "multiregional hypothesis".
Mitochondrial Eve is the name given by researchers to the woman who is the matrilineal common ancestor of all living humans. Eve was a member of a population of humans around 150,000 years ago in Africa. We know about Eve because of mitochondria organelles that are only passed from mother to offspring. Each mitochondrion contains Mitochondrial DNA, and the comparison of DNA sequences from Mt DNA reveals a phylogeny. Based on the molecular clock technique of correlating elapsed time with observed genetic drift, Eve is believed to have lived about 150,000 years ago.
Although Mitochondrial Eve was named after Eve of the Genesis creation, this has led to some misunderstandings amongst the general public. A common misconception is that Mitochondrial Eve was the only living female of her time -- she was not thought to be. It is thought that there wer many women alive at the same time as Mitochondrial Eve, but only Mitochondrial Eve produced an unbroken line of daughters that persists today -- each of the other matrilineal lineages was broken when a woman had no children or only sons.
Consider a family tree of all humans living today. Now imagine a line from each individual to their mother, and continue those lines from each of those mothers to their mothers, and so on. Going back through time the lineages will converge as sisters share the same mother. The further back in time one goes, the fewer lineages there will be until only one lineage is left -- this is the common matrilineal ancestor of all the humans we started with, i.e. Mitochondrial Eve.
The smaller a population, the more quickly matrilineal lineages converge.
We know about Eve because of mitochondria organelles that are only passed from mother to offspring. Each mitochondrion contains Mitochondrial DNA (MtDNA), and the comparison of DNA sequences from MtDNA reveals a molecular phylogeny. Because of recombination, genes in nuclear DNA become mixed and therefore we can be is statistically less certain about their origins. Mutations that reveal, and thus diversity is magnified in mitochondrial DNA. Population bottlenecks are particularly magnified (Wilson et al 1985).
As mitochondria are inherited matrilineally, Y-chromosomes are inherited patrilineally. Thus it is possible to apply the same principles outlined above to men. The common patrilineal ancestor of all humans alive today has been dubbed Y-chromosomal Adam. Importantly, he was not alive at the same time as Mitochondrial Eve but much more recently, and certainly was not her husband.
Allan Wilson, Rebecca Cann, Steven Carr, M. George Jr., U. B. Gyllensten, K. Helm-Bychowski, R. G. Higuchi, Stephen Palumbi, E. M. Prager, R. D. Sage, and Mark Stoneking laid out the theoretical background for the analysis of mitochondrial DNA in 1985 paper. Cann, Wilson and Stoneking then proposed the concept of Mitochondrial Eve in a 1987 paper in Nature. Cann et al used restriction mapping on 147 persons from five separate populations to derive their data. Gradually mitochondrial DNA sequence data from more people around the world were collected, giving a better picture.
Eve and the Out-of-Africa theory
Mitochondrial Eve is sometimes referred to as African Eve, an ancestor who has been hypothesized on the grounds of fossil as well as DNA evidence. According to the most common interpretation of the mitochondrial DNA data, the titles belong to the same hypothetical woman. Family trees (or "phylogenies") constructed on the basis of mitochondrial DNA comparisons show that the living humans whose mitochondrial lineages branched earliest from the tree are indigenous Africans, whereas the lineages of indigenous peoples on other continents all branch off from African lines. Researchers therefore reason that all living humans descend from Africans, some of whom migrated out of Africa to populate the rest of the world. If the mitochondrial analysis is correct, then because mitochondrial Eve represents the root of the mitochondrial family tree, she must have predated the exodus and lived in Africa. Therefore many researchers take the mitochondrial evidence as support for the "single-origin" or Out-of-Africa model.
The construction of family trees from DNA data is an inexact science, however. Critics of the "African genesis" model argue that the mitochondrial evidence can be explained as well or better by trees that associate Eve most closely to the indigenous peoples of Asia. As of 2003, however, following advances in computing power and in methods of tree determination, these criticisms have diminished. In any event, the strongest support that mitochondrial DNA offers for the African-origin hypothesis may not depend on trees. One finding not subject to interpretation is that the greatest diversity of mitochondrial DNA sequences exists among Africans. This diversity would not have accumulated, researchers argue, if humans had not been living longer in Africa than anywhere else. Analysis of Y chromosome sequences have corroborated the evidence that mitochondrial DNA has provided for an African origin for hominids.
In popular culture
Bryan Sykes has written a popular science book entitled The Seven Daughters of Eve.
The Discovery Channel has produced a documentary entitled The Real Eve.
Y chromosome Adam
In human genetics, Y-chromosomal Adam (Y-mrca) is the male counterpart to mitochondrial Eve: the last male ancestor from whom all male human Y chromosomes are descended. Unlike other genes, those of the Y chromosome are passed exclusively from father to sons, just as mitochondrial DNA is passed to all children only by their mothers.
The Y-chromosomal Adam is the last male ancestor of all humans, tracing only through the male line, through fathers, paternal grandfathers, etc.
Y-chromosomal Adam hypothetically is not the same individual at all points in human history. The last male-line-only ancestor of humans alive today is hypothetically different from the one for humans alive a thousand years in the future: as male lines die out, a more recent individual, the Y-mrca of a subtree of the preceding Y-Adam, becomes the new Y-Adam.
The Y-chromosomal Adam for living humans probably lived between 60,000 and 90,000 years ago, judging from molecular clock and genetic marker studies. While their descendants certainly became close intimates, Y-chromosomal Adam and mitochondrial Eve are separated by thousands of generations. They are named after the "Adam" and "Eve" in Genesis as a metaphor only, and are not considered to be the first humans. There would have been many others alive at the same time.
Based on DNA analysis as of 2002, both Y-chromosomal Adam and mitochondrial Eve are believed to have lived in Africa, though approximately 85,000 years apart. This is part of the Out-of-Africa theory of human evolution.