Advanced ancestry

THE 9 billion humans inhabiting theearth today originated about 270,000years ago from a population of a fewthousand in one region of the world andthen dispersed, says a recent study. Thischallenges the origin of species hypothesis which says that modern humansbegan evolving about a million yearsago from a more primitive humanancestor, the Homo erectus, in differentparts of the world simultaneously.

But these findings are consistentwith the prevalent view among anthropologists about the origins of modernhumans (Homo sapiens).

On the evolutionary scale, the270,000-year date is considered close tothe 200,000-year age claimedin 1987 fora common female ancestor, who wasinevitably called "Eve". In both cases,the modern human DNA, which is considered to be a repository of evolution-ary information, was analysed.

The later study is based on thegenetic structure of Y-chromosome taken from38 men of different racialbackgrounds from all overthe world. It suggests thatour common ancestorswere probably an isolatedtribe of a more primitivehuman species which didnot mate with othergroups. Because of this,their evolutionary changesmade them different from their ancestors.

Says 'Walter Gilbert of HarvardUniversity, "What's striking about ourfinding is that when we looked at allthese guys, we didn't find any differences. If our species had evolved muchearlier, you wouldn't find that. You'dexpect differences."

The researchers did not examine theentire Y-chromosome of the subjects,but concentrated on a small segmentcontaining 729 bases of geneticsequence. Called the "intron", thissequence lies within the gene butplays no known role in the gene'scode. Robert L Dorit of the YaleUniversity, said they chose thisparticular segment hoping to find some differences if stuciied in a widevariety of men.

They reasoned that these differenceswould help construct a family tree andprovide an estimate of when the common ancestors lived. If the DNA segmentin people differs in only one or a fewbases, they are presumedmore closely related thanthose people whose segments differ in more bases.

But, to their surprise,Dorit, Gilbert and HiroshiAkashi of the University ofChicago, found no differences in the intron, eventhough they checked the DNA of men from everymajor racial and geographic region.