Remember "Mitochondrial Eve," the mother of humankind? Ourcommon female ancestor, she launched the human race between166,000 and 249,000 years ago in Africa.
That at least is the spin that population geneticists, in the late 1980s,put on the maternal founder of Homo sapiens, thereby setting off aminor flurry in the media.
"They looked at mitochondrial DNA from all around the world,"recalled molecular biologist Walter Gilbert, who shared the 1980Nobel Prize in chemistry for co-discovering DNA sequencing. "Theycould see gene mutations," Gilbert told BioWorld Today, "becausemitochondrial DNA changes more rapidly than genomic DNA."
The accumulation of mutations reflects the measured passage of timeand multiplication of polymorphisms in individuals and groups.Because mitochondria _ the "powerhouse" organelle that suppliesevery cell's energy _ comes down to a person solely from hismother, those variations sum up only maternal inheritance.
Hence, a mitochondrial "Eve," not "Adam."
Gilbert, an endowed professor at Harvard University, who teaches inthe department of molecular and cellular biology, produces thatmissing male progenitor in today's issue of Science. The paperdescribing his experiment bears the title, "Absence of polymorphismat the ZFY locus on the human Y chromosome."
"Our experiment comes in from left field," Gilbert observed,"because it starts to do the male equivalent of the mitochondriaexperiment, to see if there is a sign of a common father."
The Y chromosome, present on the human genome of men only, isthe genetic mirror image of a mitochondrion. It bequeaths a father'smaleness to approximately half of developing embryos. Beingmonoploid, the Y is a non-recombining chromosome; it has nopartner with which to mingle its DNA.
Y Chromosome: Happy Hunting Ground For `Old Adam'
To begin with, Gilbert and his co-authors assembled cross-section ofDNA samples worldwide from hair roots, cheek scrapings or sperm,donated by 38 men in 29 countries on all five continents. Theysequenced the entire 729-base-pair ZFY [zinc-finger] intron in all ofthem, for a total of 27,702.
"This gene," the Science paper notes, "appears to be involved insperm or testes maturation."
"Trying to do the same mitochondrial experiment with the Ychromosome," Gilbert said, "we expected to see mutational changes,and be able to plot population evolution. Looking at samples from allover the world," he continued, "we didn't see any changes at all. Atfirst, we were tremendously pleased and annoyed. Then we realizedthat not seeing any polymorphisms meant that we were looking at acommon male ancestor. But at first we had no idea when that was."
This immutable Y chromosome, monomorphic from Africa, Europeand Eurasia to North and South America, came as a surprise. "Welooked at the same chromosomal region in chimpanzees, gorillas andorangutans," Gilbert observed, "and saw many nucleotidedifferences."
Putting that primate plasticity together with the fossil record, he said,"we could make statements about how fast, how rapidly, we wouldexpect to see mutational changes."
Then, "by tricky Bayesian [probability theory] argument," Gilbertwent on, "we turned the fact that we haven't seen any changes into anestimate for a maximum time to go to a common male ancestor.
"That time is recent. The deepest estimate we made is 270,000 yearsago, which dovetails with the `Mother Eve' mitochondrial time, inthat it says we have a recent common ancestor."
He pointed out that this doesn't hint at where in the world that Y-chromosome founding father lived; just that his advent was veryrecent, compared to one leading interpretation of the fossil record.
On an evolutionary time-scale, the geneticists plot their genealogicalfamily trees on evolutionary "trees," which diagram how populationsubgroups branch out, and how species split off. Thus, chimpanzeesparted company with people between four and six million years ago.
"One way of looking at that mitochondrial data, which characterizedmankind's common female ancestor," Gilbert explained, "is to lookat how much variation you see in mitochondria across the humanpopulation. "You see much greater variation across Africa [signifyinga longer time frame] than you do across Europe or Asia. The splitfrom primates is in Africa; the branches [indicating human populationspread] came off separately."
Factions Within Factions Raise Objections
Those genetic archeologists who concede that conceptualmitochondrial mother in Africa took slings and arrows from twooutraged quarters. "A whole group of people challenged them ontree-building grounds," Gilbert said; "on how they tried to connectthe pattern of molecules."
Others, the physical anthropologists, attacked their interpretation ofthe fossil record. These unearthers of ancient, petrified humanremains, he added, are themselves divided into two factions.
One persuasion believes that Homo sapiens originated late inprehistory, within the time frame of mitochondrial Eve. The other,defending its "multi-regional hypothesis," maintains that the humanspecies is much more ancient, and goes back to its precursor, Homoerectus, who spread around the world a million years ago.
"Its proponents," Gilbert, said, "argue that Homo erectus turned intoHomo neanderthalis in Europe, and that Neanderthal Man thengradually changed into modern man. But," he added pointedly,"Homo erectus, according to the fossil record, did not turn intoNeanderthal in China. Rather, it turned into modern man in Chinaand in Africa."
The competing fossil hypothesis that humankind came later to thescene claims that Homo sapiens first emerged in fossils in Africasome 200,000 years ago, then migrated out of Africa to the Near Eastabout 100,000 years later, and 50,000 or 60,000 years ago surfacedin Europe, Asia and Australia, thus covering the world.
Appraising Gilbert's paper, population geneticist Svante Pbo of theUniversity of Munich wrote in Science: "So far we have just seen thesmall beginnings of what someone has called the biggestarcheological excavation of all times: the quest into the genome toreveal our past." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.