Remember "Mitochondrial Eve," the mother of humankind? Our common female ancestor, she launched the human race between 166,000 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 a minor flurry in the media.
"They looked at mitochondrial DNA from all around the world," recalled molecular biologist Walter Gilbert, who shared the 1980 Nobel Prize in chemistry for co-discovering DNA sequencing. "They could see gene mutations," Gilbert told BioWorld Today, "because mitochondrial DNA changes more rapidly than genomic DNA."
The accumulation of mutations reflects the measured passage of time and multiplication of polymorphisms in individuals and groups. Because mitochondria _ the "powerhouse" organelle that supplies every cell's energy _ comes down to a person solely from his mother, those variations sum up only maternal inheritance.
Hence, a mitochondrial "Eve," not "Adam."
Gilbert, an endowed professor at Harvard University, who teaches in the department of molecular and cellular biology, produces that missing male progenitor in today's issue of Science. The paper describing his experiment bears the title, "Absence of polymorphism at 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 mitochondria experiment, to see if there is a sign of a common father."
The Y chromosome, present on the human genome of men only, is the genetic mirror image of a mitochondrion. It bequeaths a father's maleness to approximately half of developing embryos. Being monoploid, the Y is a non-recombining chromosome; it has no partner 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 of DNA samples worldwide from hair roots, cheek scrapings or sperm, donated by 38 men in 29 countries on all five continents. They sequenced the entire 729-base-pair ZFY [zinc-finger] intron in all of them, for a total of 27,702.
"This gene," the Science paper notes, "appears to be involved in sperm or testes maturation."
"Trying to do the same mitochondrial experiment with the Y chromosome," Gilbert said, "we expected to see mutational changes, and be able to plot population evolution. Looking at samples from all over the world," he continued, "we didn't see any changes at all. At first, we were tremendously pleased and annoyed. Then we realized that not seeing any polymorphisms meant that we were looking at a common male ancestor. But at first we had no idea when that was."
This immutable Y chromosome, monomorphic from Africa, Europe and Eurasia to North and South America, came as a surprise. "We looked at the same chromosomal region in chimpanzees, gorillas and orangutans," Gilbert observed, "and saw many nucleotide differences."
Putting that primate plasticity together with the fossil record, he said, "we could make statements about how fast, how rapidly, we would expect to see mutational changes."
Then, "by tricky Bayesian [probability theory] argument," Gilbert went on, "we turned the fact that we haven't seen any changes into an estimate for a maximum time to go to a common male ancestor.
"That time is recent. The deepest estimate we made is 270,000 years ago, which dovetails with the `Mother Eve' mitochondrial time, in that 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 very recent, compared to one leading interpretation of the fossil record.
On an evolutionary time-scale, the geneticists plot their genealogical family trees on evolutionary "trees," which diagram how population subgroups branch out, and how species split off. Thus, chimpanzees parted company with people between four and six million years ago.
"One way of looking at that mitochondrial data, which characterized mankind's common female ancestor," Gilbert explained, "is to look at how much variation you see in mitochondria across the human population. "You see much greater variation across Africa [signifying a longer time frame] than you do across Europe or Asia. The split from primates is in Africa; the branches [indicating human population spread] came off separately."
Factions Within Factions Raise Objections
Those genetic archeologists who concede that conceptual mitochondrial mother in Africa took slings and arrows from two outraged quarters. "A whole group of people challenged them on tree-building grounds," Gilbert said; "on how they tried to connect the pattern of molecules."
Others, the physical anthropologists, attacked their interpretation of the fossil record. These unearthers of ancient, petrified human remains, he added, are themselves divided into two factions.
One persuasion believes that Homo sapiens originated late in prehistory, within the time frame of mitochondrial Eve. The other, defending its "multi-regional hypothesis," maintains that the human species is much more ancient, and goes back to its precursor, Homo erectus, who spread around the world a million years ago.
"Its proponents," Gilbert, said, "argue that Homo erectus turned into Homo neanderthalis in Europe, and that Neanderthal Man then gradually changed into modern man. But," he added pointedly, "Homo erectus, according to the fossil record, did not turn into Neanderthal in China. Rather, it turned into modern man in China and in Africa."
The competing fossil hypothesis that humankind came later to the scene claims that Homo sapiens first emerged in fossils in Africa some 200,000 years ago, then migrated out of Africa to the Near East about 100,000 years later, and 50,000 or 60,000 years ago surfaced in Europe, Asia and Australia, thus covering the world.
Appraising Gilbert's paper, population geneticist Svante Pbo of the University of Munich wrote in Science: "So far we have just seen the small beginnings of what someone has called the biggest archeological excavation of all times: the quest into the genome to reveal our past." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.