lpetrich

That article by evolutionary microbiologist Carl Woese has finally come out in <a href="http://www.pnas.org" target="_blank">PNAS</a>. One may need a subscription to view the full article.

Some comments:

CW starts off with some speculations about the origin of the eukaryotic cell, waving them away as beside the point, as it were. Such hypotheses generally picture the ancestral eukaryotic cell as some combination of a proteobacterium and an archaebacterium, with in some scenarios the archae living inside the proteo and becoming reduced to its genetic material with the proteo host losing its genetic material to its symbiont.

Not surprisingly, the next question is where did those ancestors come from? Which CW tries to answer. As he notes, lateral gene transfer may confuse the family tree of life beyond meaning. However, several important proteins agree with the familiar ribosomal-RNA family tree, and CW finds it significant which proteins they are -- and which proteins they are not.

Translation systems (RNA -&gt; protein) are universally distributed, and all have similar form; most, though not all, of the ribosomal proteins agree with the rRNA family tree, including the great split between Bacteria and Archaea/Eukarya.

The proteins that attach amino acids to transfer RNA's, however, are much more scrambled, most likely because they are relatively self-contained systems that are not likely to be very incompatible in a different organism.

In fact, the RNA's are the most critical part of the translation system, suggesting descent from an "RNA world".

Transcription systems (DNA -&gt; RNA), though universal, have differences that suggest that the shared ancestor was somewhat primitive.

Replication systems (DNA -&gt; DNA) appear to have been invented twice, once in Bacteria and once in Archaea/Eukarya.

So, in effect, RNA-&gt;proteins came first, then the invention of DNA as a master-copy version of RNA, and then improved DNA handling.

Much of that document may be described as theoretical, however, considering such questions of when well-defined organisms can reasonably be said to have appeared.

[ June 26, 2002: Message edited by: lpetrich ]</p>