The Kurland et al. article did not—by far—represent the consensus view of scientists working on eukaryotic origins. However, this paper has, until now, gone without much challenge in the literature (although it shows 11 citations at ISI).
Kurland et al. argued for the (early) emergence of eukaryotes prior to prokaryotes:
"...we favor the idea that the host that acquired the mitochondrial endosymbiont was a unicellular eukaryote predator, a raptor. The emergence of unicellular raptors would have had a major ecological impact on the evolution of the gentler descendants of the common ancestor. These may have responded with several adaptive strategies: They might outproduce the raptors by rapid growth or hide from raptors by adapting to extreme environments. Thus, the hypothetical eukaryote raptors may have driven the evolution of their autotrophic, heterotrophic, and saprotrophic cousins in a reductive mode that put a premium on the relatively fast-growing, streamlined cell types we call prokaryotes."In essence, complex eukaryotes came first, followed by prokaryotes which evolved by reduction. In today's letter, Martin et al. point out that this is a re-clothed view of eukaryotic evolution that was popular in the early 1980s. At the time, this idea was mostly centered on the origin and evolution of introns, which were assumed by some to be responsible for building the ancestral set of protein-coding genes by exon shuffling (the introns-early view); to be true, massive intron loss must have characterized the (reductive) evolution of prokaryotes. This scenario has since been thoroughly debunked. Although Kurland et al.'s hypothesis includes more than introns, it suffers from the same problems that led to the demise of introns-early.
In their response,
"...cellular and molecular biology, especially genomics, reveals signs of an ancient complexity of the eukaryotic cell. This new information was not available to older hypotheses for eukaryote origins..."I don't find Kurland et al.'s arguments any more compelling than when I first encountered them almost twenty years ago. However, it's great to see these important questions being discussed on the pages on Science.
Picture and caption from MSNBC (Lesley Joan Collins/Science).
"This illustration shows a single-celled predator (colored brown), swallowing up much smaller and less complex single-celled bacteria (yellow and green). Researchers say such a predatory eukaryote, nicknamed "Fred the Raptor," would have had "a major ecological impact" during the early stages of cellular evolution."