News: Life got bigger in two, million-fold leaps, scientists say

Life got bigger in two, million-fold leaps, scientists say
December 22nd, 2008 in Space & Earth science / Earth Sciences


(PhysOrg.com) -- Extremes are exciting. Does anyone really think dinosaurs
would capture our imagination the way they do if they hadn't been so huge?
You don't see natural history museums vying for fossil skeletons of
prehistoric rodents. It's the Tyrannosaurus rex fossils they salivate and
squabble over. And would the Hollywood glitterati cart around those little
teacup pups if they weren't so dang tiny and cute? Not likely.

Earth's creatures come in all sizes, yet they (and we) all sprang from the
same single-celled organisms that first populated the planet. So how on
Earth did life go from bacteria to the blue whale?

"It happened primarily in two great leaps, and each time, the maximum size
of life jumped up by a factor of about a million," said Jonathan Payne,
assistant professor of geological and environmental science at Stanford.

Payne, along with a dozen other paleontologists and ecologists at 10
different research institutions, pooled their existing databases, combed the
scientific literature and consulted with taxonomic experts in a quest to
determine the maximum size of life over all of geological time.

That might sound like a rather large undertaking, but, fortunately, the
quest was made easier because even the professionals have a fascination with
the size of the fossilized.

"The nice thing about maximum size is that people tend to remark on very
large fossils, so they are much easier to track down in the geologic
literature than anything else," Payne said.

In addition to quantifying the enormity of the two leaps in maximum size,
the researchers also pinned down when those leaps took place. Both leaps
coincided with periods when there was a major increase in the amount of
oxygen in the atmosphere.

Payne said that many researchers already recognized, in a qualitative way,
that the change in maximum size had occurred this way. "But our study really
reflects the first time that anybody has tried to quantify exactly how
stepwise it was and how big those size jumps were," he said.

A paper detailing the research by Payne and his colleagues is scheduled to
be published in the Dec. 22, 2008, online early edition of the Proceedings
of the National Academy of Sciences and is available online through
EurekAlert.

The two other principal investigators of the research group, funded through
the National Evolutionary Synthesis Center, are Michal Kowalewski of
Virginia Tech and Jennifer Stempien of the University of Colorado-Boulder.

So how did it all happen? The first fossilized bacterial cells date to
approximately 3.4 billion years ago, although life likely originated several
hundred million years before. Between 2.7 and 2.4 billion years ago,
cyanobacteria, formerly known as blue-green algae, originated and were of
particular evolutionary and geological importance because they excrete
oxygen as a waste product during photosynthesis. So far as science can tell,
they were the first and only organisms to evolve oxygen-producing
photosynthesis.

"All of the oxygen in the atmosphere ultimately exists because of the
evolution of cyanobacteria," Payne said. "Plants that produce oxygen today
during photosynthesis, their ability to do that is ultimately derived from
cyanobacteria."

Single-celled bacteria remained the largest life form on Earth, cranking out
the oxygen, until about 1.6 billion years ago. At that point, a new life
form shows up in the fossil record.

"The first jump in maximum size happens when the first eukaryotic organisms
show up as fossils," Payne said. "And those fossils are approximately a
million times bigger than anything that had come before on Earth."

Although the first fossil eukaryotes were likely also single-celled
organisms, the eukaryotes distinguish themselves by means of their internal
structure and functioning. Instead of having the cellular processes of life
take place by means of diffusion in the cell, eukaryotes have organized
innards, with a nucleus and other cellular structures that are dedicated to
specific functions in the respiratory process.

"The fossil record indicates pretty clearly that you need a eukaryotic cell
to make that first size jump," Payne said. "It isn't just that the bacteria
don't get there as fast, it is that bacteria still haven't gotten there 1.6
billion years later.

"Clearly, organismal organization matters," Payne said. "Not just at the
time the size increase happens, but it continues to be a limitation on size.

For approximately the next billion years, life on Earth stayed about the
same size, with only modest increases. Then about 600 million years ago, at
the same time as another major boost in the amount of oxygen in the
atmosphere, life leaped in size again.

This time, it was a million-fold size leap of multi-cellularity. Payne said
there are clearly multi-cellular eukaryotes in the fossil record for several
million years before this size leap, but the real explosion of size increase
didn't happen until the oxygen level bumped up.

So why do the size leaps seem to hinge on the amount of oxygen in the air?

"There are a few things that could be going on," Payne said. "The first
thing is that eukaryotic cells require oxygen for metabolism. So if they
want to take organic matter and burn it up to have energy in their cell,
they need oxygen. That sets the first and probably most important
limitation."

Payne said this limitation also applies to multi-cellular eukaryotes, which
likewise depend on extracting oxygen from the surrounding environment and
using that in their cells to obtain energy. "There is also evidence that
oxygen may mediate some other biochemical processes," he said.

As for just what triggered both the boosts in atmospheric oxygen, Payne said
that isn't quite as clear. It may be that the first jump in oxygen came
because cyanobacteria simply proliferated to the point that they were
cranking out more oxygen than could be consumed through chemical reactions
with material at Earth's surface, the only way that oxygen wouldn't have
been released back into the atmosphere in the era before oxygen breathing
creatures existed.

The possible causes of the second jump in oxygen are less clear, Payne said,
but regardless of the puzzles that remain to be sorted out, the timing and
magnitude of the jumps up in maximum size are clear. And Payne said the size
jumps applied to a vast number of species.

"Whatever is controlling this second size increase appears to operate across
many different groups. It is not something limiting one group alone," he
said. "There also appears to be an increase even in the maximum size of
groups of organisms like multi-cellular algae, so the size increase doesn't
appear to be limited just to animals."

One other question remains to be answered: Can we look forward to another
great leap in size? Will we see housecats larger than our houses?

"We've speculated on that a little bit, just sort of thinking about what if
you went up another step," Payne said.

"The next level of organization, going along this kind of theme, presumably
would be something like insect societies, where you have individual
multicellular eukaryotes that specialize in terms of what kind of function
they carry out in a larger organization of these individuals. Something like
an ant colony or a human society would be in some ways the next
organizational level.

"But, if you look at human society as an example, we use so much of the
gross primary productivity on Earth, it doesn't appear there would be room
for a lot of species at that next level of organization and maximum size. At
that point you're actually getting towards the physical size limits just
imposed by the size of our planet."

Provided by Stanford University
http://www.physorg.com/news149188848.html

Posted by
Robert Karl Stonjek


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