According to a new report, complex
cells like those in the human body probably resulted from the
fusion of genomes from an ancient bacterium and a simpler
microbe, Archaea, best known for its ability to withstand
extreme temperatures and hostile environments. The finding
provides strong evidence that complex cells arose from
combinations of simpler organisms in a symbiotic effort to
survive.
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 their
genomes to form the first eukaryote, and in the processes two
different branches of the tree of life were fused to form the
ring of life," Lake said.
The work is part of an effort supported by the National
Science Foundation--the federal agency that supports research
and education across all disciplines of science and
engineering--to re-examine historical schemes for classifying
Earth's living creatures, a process that was once based on
easily observable traits. Microbes, plants or animals were
said to be related if they shared certain, mostly physical,
characteristics. DNA technology now allows much closer
scrutiny of hereditary molecules, which provides a more
accurate and detailed picture of the genetic relationships
between and among living things.
"New computational tools and comparative analyses will
undoubtedly find instances in which the evolutionary record
will need to be set straight," said James Rodman, a program
officer in NSF directorate for biology, which funded the
research. "This new fellowship among microbiologists,
evolutionists, and computationalists will provide a much
fuller picture of the relatedness of living things."
Lake and Rivera analyzed and compared the genomes of 30
microorganisms selected from the three categories (eukaryotes,
bacteria and Archaea). All of the microbes contained about the
same number of genes. The researchers then used the computer
to produce genome combinations that reflected the most likely
ancestors of modern eukaryotes. Their analysis, they say,
showed that two ancient prokaryotes--one most similar to a
bacterium, and one an Archaea--combined genomes out of a
mutually advantageous need to survive.
That theory, known as endosymbiosis, has been a popular
explanation of how eukaryotic cells acquired smaller
components to carry out cellular processes. According to the
report, modern eukaryotes obtained genes required to operate
the cell from the bacterial side of the family, and the
information-carrying genes from the Archaea side.
Further, the authors say, the work also sheds light on the
"horizontal" transfer of genes--sideways from organism to
organism, rather than from parent to offspring.
###
The U.S Department of Energy, the National Institutes of
Health, and NASA also supported this work.
NSF Media Contact: Leslie Fink, 703-292-5395, lfink@nsf.gov
UCLA Media Contact: Stuart Wolpert, 310-206-0511,
stuartw@college.ucla.edu
Images available:
http://www.nsf.gov/od/lpa/newsroom/pr_all_img.cfm?ni=15000000000117
NSF PR04-114
NSF Program Officer: James Rodman, 703-292-8481,
jrodman@nsf.gov
Principal Investigator: James Lake, 310-825-2546,
lake@mbi.ucla.edu
Additional info:
http://genomics.ucla.edu/
http://www.nsf.gov/bio/deb/deb_sbbi.htm
http://www.nsf.gov/bio/atol/tol.pdf
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