Jim Lake and Maria Rivera, at the University of California-Los
Angeles (UCLA), report their finding in the Sept. 9 issue of the
journal Nature. Scientists refer to both bacteria and Archaea as
"prokaryotes"--a cell type that has no distinct nucleus to
contain the genetic material, DNA, and few other specialized
components. More-complex cells, known as "eukaryotes," contain a
well-defined nucleus as well as compartmentalized "organelles"
that carry out metabolism and transport molecules throughout the
cell. Yeast cells are some of the most-primitive eukaryotes,
whereas the highly specialized cells of human beings and other
mammals are among the most complex. "A major unsolved question
in biology has been where eukaryotes came from, where we came
from," Lake said. "The answer is that we have two parents, and
we now know who those parents were." Further, he added, the
results provide a new picture of evolutionary pathways. "At
least 2 billion years ago, ancestors of these two diverse
prokaryotic groups fused the