The Making of a Eukaryote
SOME 3.5 BILLION YEARS AGO, while bacteria were beginning to cover the world with triumphantly successful colonies, an obscure offshoot started to evolve in a strange direction that would most likely have appeared to an extraterrestrial visitor as totally aberrant in the context of life on Earth as it was then, and leading nowhere. In reality, "nowhere" fanned out, more than two billion years later, into the immensely varied groups of protists, plants, fungi, and animals, including humans--virtually the whole visible part of the biosphere. The way to this extraordinary diversity of living forms passed through a new type of cell entirely different from any known bacterium, past or present. Called eukaryotic because it possesses an authentic nucleus, this type of cell has many other characteristics that distinguish it clearly from the bacteria, or prokaryotes.
The nature of the obscure originator of the eukaryotic line is uncertain. Most of the evidence points to a prokaryote that detached from the archaebacterial branch after the first forking of the tree of life. In seeming conflict with this possibility, eukaryotes possess a few traits that appear to be derived from an ancient eubacterium. These anomalies have been variously attributed to convergent evolution, to horizontal gene transfer, and even, as proposed by the German investigator Wolfram Zillig,1 to a primeval fusion event between an archaebacterial and a eubacterial partner. The apparent mixed ancestry of eukaryotes has also been used in support of more radical proposals that picture the primitive eukaryotic ancestor as a cell anterior to prokaryotes and endowed with either DNA genes or even RNA genes.2
In this chapter, I shall assume that the eukaryotic branch issued from an ancestral prokaryote, presumably of archaebacterial nature with some admixture of