The world’s scientific community has been crunching the numbers on the animal kingdom's sizes and shapes, and have found that humans differ from each other far less than most species. The reason why is a mystery. "We don't have an answer. We have this interesting observation, but the explanation is an open hypothesis," said evolutionary biologist Andrew Hendry of McGill University. Hendry and Queens University biologist Ann McKellar combed through the scientific literature on body size and length in more than 200 species, from insects to fish to birds and, of course, humans.
They asked how humans compare to other animals in terms of body size variation. The team quantitatively compare levels of variation in body length (height) and mass within and among 99 human populations and 848 animal populations (210 species) and found that humans show low levels of within-population body height variation in comparison to body length variation in other animals. The further theorized that humans have evolved on a rugged adaptive landscape with strong selection for optimum body height variations that differ among locations.
All life on Earth all sprang from the same single-celled organisms
that first populated the planet, so how did life grow in size from
bacteria to homo sapiens 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. 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.
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?
“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.”
Posted by Casey Kazan.
http://www.mcgill.ca/newsroom/news/item/?item_id=108985
http://news-service.stanford.edu/news/2009/january7/oxysize-010709.html
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