Mungo Man: the last of his kind?

By LEIGH DAYTON
09jan01

SIX Ts, two Gs and a pair of Cs: these 10 letters are about to trigger an international scientific ruckus that's likely to rage for years.

Why? Well, T,G and C are abbreviations for Thymine, Guanine and Cytosine. Along with Adenine, these substances form the "letters" of the "book of life", deoxyribonucleic acid. DNA for short. Strung together, those 10 letters form part of a "word", a gene, never before seen by science.

And it's a gene from a human being, a man who lived and died in Australia. So who was he? And why are researchers already arguing passionately about him and his relatives?

The story began in the dreamtime. A small-boned man died near the shores of Lake Mungo, where his people had been fishing for perch and hunting the plentiful game that came to the water to drink. His body was carefully positioned in a grave -- hands crossed over pelvis -- sprinkled ceremonially with powdered red ochre, and covered with earth.

Fast forward to the 20th century. In 1974, University of Melbourne geomorphologist Jim Bowler was exploring the now dry lake bed, one of several in the Willandra Lakes region of south-western NSW. He spotted a bone. Thinking it might be from a large extinct animal, Bowler called for a scientific second opinion.

Eventually, a group of experts in anthropology and prehistory gathered at Lake Mungo for a first-hand look. The remains found there were human, they concluded, and they were old.

So a team set to work to excavate the prehistoric treasure trove. And one long working day, Alan Thorne, a young anthropologist with the Australian National University (ANU), completed the excavation of the remains of the man, laid to rest so long ago. But just when was "long ago"? Radiocarbon dating soon revealed that "Mungo Man" -- LM3 in science speak -- had died at least 28,000 years ago, possibly as many as 32,000 years ago.

The news hit like a bombshell. For scientists and the Australian public were just getting used to the idea that Aborigines had lived in Australia for more than a few thousand years, as originally believed. There was even more buzzing, come the mid-1980s, when improved radiocarbon techniques pushed back LM3's date to at least 38,000 years before present (BP).

But in 1999, LM3 hit the international headlines, big time. ANU researchers had used three separate state-of-the-art dating methods to show that he had probably lived between 56,000 and 68,000 BP, most likely 60,000 BP.

That more than trebled the date for humanity's first arrival on the continent, and set scientific tongues wagging.

Some experts, including Bowler, claimed the dates were wrong, or, at best, uncertain. Others, like ANU physical anthropologist Colin Groves, welcomed the dates, arguing that they supported his theory of the origins of humanity, as opposed to one backed by Thorne.

For a time, it seemed that Groves and his compatriots, worldwide, were winning the debate. Data was emerging from genetics laboratories, here and abroad, which were based on detailed analysis of the DNA of living people. It lent weight to their Out of Africa model.

That model proposes that proto-people evolved in Africa, and spread out from there, beginning about 1.5 million years ago. Then, between 100,000 and 150,000 BP, a group of fully modern humans appeared on the scene. They swept out of their African homeland, replacing the less able populations already established in Europe and Asia.

Now, however, Mungo Man's surprising 10-letter word -- literally whispered from the grave -- may have given the edge to the position held by Thorne and company.

In a nutshell, their Regional Continuity model agrees that, yes, archaic people began migrating out of Africa about 1.5 million years ago. But instead of being supplanted by new, improved humans -- who went on to colonise the planet -- those first "almost" people continued, together, down the evolutionary path to full human status. They did this with a little hanky-panky -- that is, by interbreeding with one another. Eventually, we evolved from them ... or so the theory goes.

Mungo Man helps make the case for Regional Continuity for two reasons, argues Thorne. First, all the experts agree that his skeletal remains are absolutely modern.

In fact, he looks more modern than six other ancient Aborigines, all well-dated to more recent times. Second, this anatomically modern man possessed extinct genetic material.

What this means, Thorne reasons, is that at least one group of proto-people, who lived outside Africa, evolved into human beings like us, although their genetic lineage became a dead end. Given this, Thorne says, the Out of Africa boosters can no longer argue that modern people emerged only in Africa.

Humans were always one species, busy evolving in slightly different ways, in different parts of the world. At the end of the evolutionary day, some genes were passed on; others not.

This may seem baffling, but the argument becomes clearer after a closer look at where those critical Ts, Gs, and Cs came from. They came from a part of LM3's cells known as mitochondria. The mitochondria are the powerhouse of cells. They produce the energy the body's cells need to do their job, and there are many of them inside each and every cell. In turn, all mitochondria contain a single gene that controls the little powerhouse itself.

Because of a quirk of nature, mitochondrial DNA (mtDNA) is only passed down the female line.

This means that although Mungo Man's mtDNA lineage is extinct, there is a possibility that some of his other 100,000 or so genes were passed on. If so, his clan may have contributed to the evolutionary line that led to modern people.

That's all up for grabs, scientifically, and will be hotly debated in the months and years to come.

But not only was LM3's mtDNA totally unknown to science, it contained another breathtaking surprise for the researchers. They noticed that a small bit of his mtDNA resembled a so-called "insert", a tiny stretch of bases often found on chromosome 11 in the nuclear DNA of contemporary people.

Previous studies of the prevalence and distribution of the peculiar sequence, done by other scientists, had suggested that the insert was a relic, deep from our evolutionary past. It must be quite old, they estimated, older than the most recent common ancestor of all living people. Yet no one knew just how old, or where the genetic relic came from. Had the Australians, purely by chance, stumbled upon the source of the mysterious insert?

It's quite possible, says team member Simon Easteal, an evolutionary geneticist at ANU's John Curtin School of Medical Research.

"The sequence from LM3 is related to the insert," he says.

"This suggests that someone related to LM3 was the source of the insert, which moved into the nuclear DNA of another person at some point in time. We can't say where that first person lived, probably in Asia, New Guinea, or possibly Australia."

Easteal should know. He analysed all the sequences his colleagues extracted from the ancient Aborigines. What's more, he compared them, one to another, and to matching sequences from 45 Aborigines living near Darwin or the Riverina district around Willandra lakes.

Easteal then added sequences from 3453 people from around the world, tossed in the new Neandertal data, and, while he was at it, comparable sequences from chimpanzees and bonobos, or pigmy chimps.

After clusters of five computers calculated 24 hours a day, seven days a week, for two weeks, Easteal had a genetic tree, showing the most likely "pattern of descent" of all the people in the data base.

It goes like this: first to branch off the trunk of our evolutionary tree were the two species of chimps; next the Neandertals; then a line leading to LM3 and the "gene insert" people; and finally the lineage that led to the most recent ancestor of contemporary people, including the ancient Australians, except LM3.

There was a hint that, like LM3, one of the ancient Aborigines from Kow Swamp might have come from an early-diverging lineage. The data was just too sketchy to be confident.

According to the new findings, then, modern people arrived in Australia before the ancestors of all of us living today. But what happened to Mungo Man and his clan? If his lineage is extinct, surely he is too. Maybe yes; maybe no. That's part of the scientific dust-up about to engulf Thorne and his colleagues.

Recall that the sticking point is the fact the gene that the Australians plucked from Mungo Man's bones is transmitted only by women. The picture will become clearer when DNA technology takes the next leap forward, enabling researchers to study nuclear genes that we get from both parents. For now, all that can be said for certain is that the women in his clan did not pass on the gene to their children. It could have been chance. Maybe only the men in his line had children, or perhaps something more malevolent happened.

Dastardly deeds would not surprise John Mitchell, a human geneticist at La Trobe University. Like Groves, he disagrees with Thorne's assertion that LM3 puts Regional Continuity ahead in the theoretical race to explain our origins. Instead, the findings suggest to Mitchell that fully modern people began trickling out of Africa earlier than previously believed. That trickle peaked about 100,000 to 150,000 BP.

According to Mitchell, Mungo Man's ancestors probably left early, travelled east, and eventually reached Australia ... where they met their end.

"I would not find it strange that humans who left Africa (after LM3's ancestors) did replace -- by interbreeding or killing -- anatomically modern humans, or any other prior forms," Mitchell says.

"We do it today. I don't see it strange that we did it 100,000 years ago." If so, this must be the oldest crime ever solved by DNA.

But do the six Ts, two Gs and that pair of Cs hold the answer to the bigger mystery of the origins of humanity? Perhaps. The scientific jury is just beginning its deliberations. Don't expect a verdict for some time to come.

Source: http://news.com.au/common/story_page/0,4057,1590695%255E11011,00.html