News: And the first animal on Earth was a...

And the first animal on Earth was a...

A new study mapping the evolutionary history of animals indicates that
Earth's first animal--a mysterious creature whose characteristics can only
be inferred from fossils and studies of living animals--was probably
significantly more complex than previously believed.

The study, which was funded by the National Science Foundation (NSF), is the
cover story of the April 10, 2008 issue of Nature. Using new high-powered
technologies for analyzing massive volumes of genetic data, the study
defined the earliest splits at the base of the animal tree of life. The tree
of life is a hierarchical representation of the evolutionary relationships
between species that was introduced by Charles Darwin.

Shaking Up the Tree of Life

Among the study's surprising findings is that the comb jelly split off from
other animals and diverged onto its own evolutionary path before the sponge.
This finding challenges the traditional view of the base of the tree of
life, which honored the lowly sponge as the earliest diverging animal. "This
was a complete shocker," says Dunn. "So shocking that we initially thought
something had gone very wrong."

But even after Dunn's team checked and rechecked their results and added
more data to their study, their results still suggested that the comb jelly,
which has tissues and a nervous system, split off from other animals before
the tissue-less, nerve-less sponge.

The presence of the relatively complex comb jelly at the base of the tree of
life suggests that the first animal was probably more complex than
previously believed, says Dunn.

While cautioning that additional studies should be conducted to corroborate
his team's findings, Dunn says that the comb jelly could only have achieved
its apparent seniority over the simpler sponge via one of two new
evolutionary scenarios: 1) the comb jelly evolved its complexity
independently of other animals, after it branched off onto its own
evolutionary path; or 2) the sponge evolved its simple form from more
complex creatures--a possibility that underscores the fact that "evolution
is not necessarily just a march towards increased complexity," says Dunn.
"This scenario would provide a particularly dramatic example of that
principle."

How Old is Old?

How long ago did the earliest comb jelly diverge" "Unfortunately, we don't
have fossils of the oldest comb jelly," laments Dunn. "Therefore, there is
no way to date the earliest jelly and determine when it diverged."

After diverging from other species, the comb jelly probably continued to
evolve, says Herendeen. Therefore, today's comb jelly--a common
creature--probably looks very different that did the earliest comb jelly.

Moreover, the tentacled, squishy but bell-less comb jelly developed along a
different evolutionary path than did the classically bell-shaped jellyfish,
says Patrick Herendeen, an NSF program director. Such divergences mean that
"the jellyfish type of body form has independently evolved several times,"
says Herendeen.

Remaining Gaps in the Tree of Life

While reversing the evolutionary order of the sponge and comb jelly, Dunn's
study also resolved some long-standing questions about other species. Among
these was whether millipedes and centipedes are more closely related to
spiders than to insects. The answer: spiders.

But despite these and other important evolutionary insights provided by
Dunn's team, the tree of life remains a work in progress. "Scientists
currently estimate that there are a total of about 10 million species of
organisms on earth," says Dunn. "But so far, only about 1.8 million
species--most of which are animals--have been described by science. Very few
of these species have, so far, been positioned in the tree of life."

A Methodological Breakthrough

But at least some of the tree of life's remaining gaps will likely be filled
through the use of high-powered analytic approaches pioneered in Dunn's
study--which involved using more than 100 computers to analyze more data
than incorporated into any previous comparable evolutionary study. "Dunn's
high-powered approach is just what we need to continue assembling the tree
of life," says Herendeen. "We are going to see a lot of this approach in the
future."

Dunn explains one of the advantages of his team's approach: "Even though we
looked at fewer than 100 species, they were sampled in such a way that they
inform the relationships of major groups of animals relative to each other.
Therefore, this study, and others like it, will have implications for the
placement of far more species than just those that are sampled."

Remaining Challenges

But no matter how many high-tech analytic tools scientists use to analyze
the genetics of organisms, they must still conquer "the exact same
challenges that naturalists faced 200 years ago," says Dunn. "We still don't
even know enough about many species to have a good idea where to look for
them."

"And even as it is getting easier and cheaper to analyze the DNA of
organisms with increasingly powerful computers, it is getting more expensive
and difficult to find, collect, and identify organisms." For example, Dunn's
team had to use remotely operated underwater vehicles to collect one of the
comb jellies included in this study.

Dunn concludes: "It may come as a surprise to some that the many that huge
advances in technology actually bring us right back to the same challenges
that naturalists faced 200 years ago: the day-to-day practical challenges of
just figuring out what lives on our planet, where to find it and how to
collect it."

Source: National Science Foundation
http://www.physorg.com/news127055240.html

Comment:
Pre-Darwinian 'trees of life', such as Lamarck's tree, were not called
'trees of life'. Indeed, Lamarck's branched down from the top rather than
up from the bottom as Darwin's. But to say that Darwin introduced the tree
of life is entirely false unless one is referring only to the nomenclature
associated with the branching hierarchy of the evolution of species.

Posted by
Robert Karl Stonjek


.

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