TITLE:  A New Tree of Life
      AUTHOR:  MENON, SHANTI
     JOURNAL:  Discover
    CITATION:  June, 1996, 17: 37.
        YEAR:  1996
    PUB TYPE:  Article
 IDENTIFIERS:  FOSSIL RECORDS; MOLECULAR EVOLUTION; BACTERIA; EVOLUTION; 
               MUTATION RATES; EUKARYOTES; ARCHAEA
    ABSTRACT:       By a billion years or so after Earth formed, life had 
               already taken hold. Microfossils found in 3.5 billion-year-
               old rocks in Australia show that the first living things were 
               prokaryotes, like today's bacteria, with DNA floating freely 
               in their cells. For the next 3 billion years--until larger 
               life forms evolved--the fossil record is sparse.
                    Many biologists, however, believe that life split into 
               two branches more than 3 billion years ago. That split was 
               between bacteria and archaea--bacteria-like organisms that 
               still exist today. Eukaryotes, with DNA packed in a nucleus, 
               branched off from the archaea later and gave rise to all 
               other life forms, from amoebas to people. Or so the 
               conventional view holds. Russell Doolittle, a molecular 
               evolutionist at the University of California at San Diego, 
               believes that view is flawed. He has found evidence that the 
               split between bacteria and all other life occurred much 
               later, probably as recently as 2 billion years ago.
                    Doolittle's results have attracted criticism. "It's 
               ridiculous," says Norman Pace, a microbiologist at Indiana 
               University. The 3.5 billion-year-old Australian fossils, he 
               says, look like cyanobacteria, a type of photosynthesizing 
               bacteria, and the RNA family tree indicates that 
               cyanobacteria were not the first organisms. "The archaea, the 
               eukaryotes, and most of the bacterial lineages had already 
               been generated by the time cyanobacteria were invented," says 
               Pace. Moreover, Doolittle's extrapolation of a mutation rate 
               back 2 billion years, about three times the original length 
               supported by the fossil record, strikes Pace as untenable. 
               "The evolutionary clock is not linear," he says. "Evolution 
               rates differ among different organisms, and at different 
               times during the course of their evolution. There is simply 
               no way you can extrapolate it." Doolittle claims that with a 
               large data set covering a long time, all the fits and starts 
               of evolution even out.