By Paul Mason
Scientists have had trouble reconciling data from analyses of human mitochondrial DNA and the male Y chromosome. Analyses of human mitochondrial DNA indicate that we all share a common female ancestor 170,000 years ago. Analyses of the Y chromosome indicate that we share a common male ancestor 59,000 years ago (Thomson et al. 2000). How can we account for the idea that our common grandmother is 111,000 years older than our common grandfather? Have we found evidence for the world’s oldest cougar, or is there a hypothesis (other than blaming it on statistical anomalies) that could potentially reconcile these two dates? Perhaps we are given a clue in recent findings that a small percentage of human DNA is Neanderthal. Against popular belief (NOVA), Neanderthals did not go extinct without contributing somehow to the gene pool of modern humans.
Sexual reproduction is successful because the process of chromosomal exchange and gamete fusion provides genetic variability between individuals. Asexual reproduction is the kiss of death in the long run due to deleterious mutations. Strangely enough though, inside each cell of our bodies there is a tiny energy regulating organelle that reproduces asexually. This symbiotic bacterium is vital to cellular function and is called a mitochondrion. Both boys and girls inherit their mitochondrial DNA exclusively from their mother.
In female Homo sapiens, the oocyte remains dormant in dictyate from the moment of formation in late foetal life until just prior to ovulation, thereby protecting itself from mutations in both the mitochondrial and nuclear DNA. The male germ cells on the other hand are in a ferment of mitotic and meiotic activity from puberty onwards with most spontaneous DNA mutations occurring in the testis (Short, 1997). Sperm are dependent on maternal mitochondrial DNA in the midpiece sheath for their motility, but these mitochondria are destroyed by the oocyte immediately after fertilization, so the fertilized egg contains only maternal mitochondrial DNA.
From studies of mitochondrial DNA published in Nature (Cann, Stoneking, & Wilson, 1987), population geneticists discovered that people alive today share a common female ancestor anywhere up to 200,000 years ago (most estimates are somewhere between 150,000 to 170,000 years ago). Studies of mitochondrial DNA from Neanderthals and humans have shown no indication that humans have a female Neanderthal ancestor (Ovchinnikov & Goodwin 2001; National Geographic, 2008).
Just this year, researchers have estimated that gene flow from Neanderthals to humans occurred between 80,000 and 50,000 years ago (ScienceDaily May, 2010). Researchers have long wondered if Neanderthals were an entirely separate species, and recent evidence suggests that they probably weren’t. (Actually, one of the problems teaching human evolution is that we use a Linnaean system of classification with a Buffonian definition of species—two incompatible systems). However, even if Neanderthals were a separate species, speciation without any loss of hybrid fertility is possible.
Take the example given to me by Professor Roger Valentine Short: the Camelidae that originated in Florida (The Atlantic, 1999).
The little ones migrated into South America and up into the Andes to become the Llama, Alpaca, Vicuna and Guanaco—phenotypically quite different species, but all of which will produce fertile hybrids when crossbred. The big ones migrated up the Rockies, across the Behring straits, through Mongolia and Northern China—where we find the two-humped Bactrian camel—and into India and from there into Persia and Saudi Arabia—where we find the one-humped Dromedary camel. The spread of the Camelidae from the Americas to the Middle East is an example of speciation in a sexually reproducing species as a result of reproductive isolation. However, there has been no loss of hybrid fertility. Researchers have been able to produce Camas by inseminating Alpacas with Dromedary semen. Interestingly, the reciprocal cross gave fetuses, but no liveborn young.
Modern humans may in fact be hybrids. Since Old World and New World Camelids are some 10 – 12 million years apart, we can be pretty certain that Homo neanderthalensis and Homo sapiens were able to hybridize. However, we must remember that studies have not shown any evidence of mitochondrial DNA from Neanderthals in humans (Potts & Short, 1999:59). Studies have shown though that modern humans share a common male ancestor who lived 59,000 years ago. Could this male ancestor have been Neanderthal? Indeed, the date of our closest common male ancestor correlates well with estimations of gene flow between Neanderthals and humans around 50,000 to 80,000 years ago. If H.neanderthalensis and H.sapiens were able to mate, then it is plausible that only the male H.neanderthalensis and the female H.sapiens were able to produce fertile offspring. The reciprocal cross may not have been successful.
According to Haldane’s law, the heterogametic offspring of interspecific hybrids are likely to be absent, rare, or sterile (Short, 1997). If Haldane’s Law applied to the offspring of H.neanderthalensis and H.sapiens, we would expect to find female hybrids quite commonly, but male hybrids much more rarely. The male hybrids would have carried a Y chromosome very similar to that of the original hybridizing male. The lack of Neanderthal mtDNA suggests that the initial hybridization involved a Neanderthal male, but there would probably have been few if any male hybrid offspring, so the Neanderthal Y chromosome and the mtDNA would have been quickly lost. Nonetheless, the Neanderthal autosomes would have happily mingled and interchanged with human autosomes, eventually losing their identity in the process.
Could it be that Homo neanderthalensis males were able to mate with Homo sapiens females but that the reciprocal cross was unsuccessful? Alternatively, were male H.sapiens disastrously incapable of wooing the physically more powerful H.neanderthalensis females? Or were H.neanderthalensis females simply unable to give birth to hybrid offspring? Perhaps male H.neanderthalensis outcompeted early male H.sapiens and eventually the male Neanderthal genes gained dominance (and maybe H.sapiens females somehow out-competed H.neanderthalensis females for partners). All of these possibilities potentially explain how we share a common male ancestor 59,000 years ago, but a common female ancestor 170,000 years ago. Simultaneously, these hypotheses explain why comparisons of DNA sequences in mitochondrial DNA from Neanderthals and modern humans have indicated that there was no interbreeding between these two exceedingly similar species (Potts & Short, 1999:59). Mitochondrial DNA from Neanderthals simply may not have made it into the modern human lineage. The nuclear DNA of Neanderthal males, however, possibly did.
Paul Mason is a doctoral candidate in anthropology at Macquarie University. He is currently finishing his dissertation on the relation between music and movement, and the implications for cultural evolution, in fight dances in Indonesia and Brazil. When he is musing about evolution, he is not working on his dissertation. [Greg: PAUL! Get back to your grindstone!]