[meteorite-list] Map of Life on Earth could be used on Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed May 11 16:15:10 2005
Message-ID: <200505112014.j4BKEV511003_at_zagami.jpl.nasa.gov>

http://news-info.wustl.edu/tips/page/normal/5152.html

Map of life on Earth could be used on Mars

Finding the 'peculiar' ancestor

By Alison Drain
Washingon University in St. Louis
May 9, 2005

A geologist from Washington University in St. Louis is
developing new techniques to render a more coherent story of how
primitive life arose and diverged on Earth - with implications for Mars.

Carrine Blank, Ph.D., Washington University assistant professor of earth
and planetary sciences in Arts & Sciences, has some insight concerning
terrestrial microbes that could lead to provocative conclusions about
the nature of life on Mars and other planets.

Blank approaches the task by resolving phylogenetic trees. These trees,
based upon genetic sequencing data, trace the genetic relationships
between what we think of as primitive organisms through trait
development. The relationships between early forms of life can
illuminate the relationships between organisms present on Earth today -
which fossil evidence and a method called isotopic fractionation have
failed to show conclusively.

Blank most recently presented her research at the 2004 annual meeting of
the Geological Society of America.

Haves and have-nots

Microorganisms can be divided into haves and have-nots: cells of
eukaryotes contain a nucleus, while prokaryotic organisms cells do not.
Prokaryotic organisms encompass archeal and bacterial domains of life.
Archeal organisms diverge further into euryarcheota and Crenarcheota
lineages. By piecing together genetic sequences of the three types of
prokaryotic organisms, Blank creates a genetic flow chart, which can be
interpreted to trace the appearance of environmental adaptations across
billions of years of evolution.

Genes are inherited from parents, but can transfer from one organism to
another without reproducing by a process called lateral gene transfer.
Modular metabolic genes, which are not critical for cell production,
account for most lateral gene transfers between microbes.

"There is a lot we're beginning to understand in terms of bacterial
evolution that is still not quite clear," Blank said. "What we're trying
to resolve is the evolutionary history of the core of the bacterial
cell. The core is that which is not undergoing this lateral gene
transfer, or does it extremely rarely."

Jumping genes

Jumping genes may be a headache for researchers, but they serve an
important ecological purpose, helping other organisms to succeed in
their habitats, and can illuminate trait development across the tree of
life.

"We try to construct the core with gene sequences, and then we look at
the distribution of traits such as those involved in metabolism by
laying it onto the tree," she said.

Timely appearances of certain traits among prokaryotes on the tree of
life can betray a trend of habitat divergence, facilitated by lateral
gene transfer. The emergence of traits corresponding to measurable
changes in the known geologic record allow researchers to date organisms
with relative certainty. Blank can then use chronological data to
analyze niche specialization, "where these organisms like to grow,"
among members of each life domain over geologic time.

Habitat divergence among bacteria is consistent with patterns of
divergence among the other prokaryotes, Blank's research shows. She
notes a pervasive trend of cyanobacterial organisms diverging from
low-salinity environments into marine environments over time.

"We have the ancestral Archeae - it diverges into two major lineages,
the Crenarchaeota and the Euryarchaeota, one which grows in marine
environments, the other on continents," Blank said. "They grow and
diverge for perhaps a billion years, and then they start colonizing each
other's environments. The marine Euryarchaeota eventually colonize the
terrestrial environments and the Crenarchaeota colonize the marine
environments. My point is that it could have taken a very long time for
them to come back and to form even more complex ecosystems. So the
literal interpretation of these patterns is that early habitat
specialization could have lasted for a billion years."

After mapping early habitat divergences onto the tree, Blank observes
that the ancestors of each of the three kinds of prokaryotes inhabited
one of Earth's three types of hydrothermal systems, which include
sulfurous steam vents like those which smatter the Yellowstone caldera,
hydrothermal deep-sea vents, and boiling silica-depositing springs.

"Is it a coincidence, then, that we have three hydrothermal habitats and
three major groups of prokaryotes? We aren't sure," she said. "This
could suggest that we have some really ancient habitat specialization.
These lineages specialize in these three habitats, and diverge in these
habitats for many hundreds of million years before they start moving
into other types of habitats."

The 'peculiar' ancestor

It isn't clear why bacteria diversified later, though environmental
changes, like periods of global glaciation nicknamed "snowball Earth,"
could have provided the impetus that demanded microbial adaptation.
Whatever the cause, new adaptive microbial traits can be very different
from those of their "peculiar" ancestors. It seems that, on some level,
humans and bacteria can relate.

"If we see these major patterns of divergence on Earth, we should expect
to see similar patterns on life on Mars, that is, if life ever existed
there," Blank said. "Not the same patterns, because Mars has had a
different history, but we should see trends that are analogous. You
would expect to see a peculiar ancestor specialized to a unique niche,
eventually diverging into descendants that have very different traits
than their ancestor did. These descendents would have adapted to changes
that would've happened in Mars' history."
Received on Wed 11 May 2005 04:14:30 PM PDT