It turns out that before the Cambrian explosion that Earth was oxygen poor. Thus evolution was slow and restricted to single cell life. That changed through extensive geological processes and the rate of evolution increased five fold to produce the Cambrian explosion.
That ultimately led to a virtuous cycle that maintains our modern levels of oxygen.
After saying that though, until now everyone has accepted an oxygen level of forty percent which is radically different than what this work has shaken loose. A lot of text books and papers will turn out to be seriously flawed and we must be awake to just that..
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Darwin's Dilemma' May Be Solved
Scientists following two different lines of evidence have just published
research that may help resolve "Darwin's dilemma," a mystery that
plagued the father of evolution until his death more than a century ago.
Biologists and geologists have been puzzled for decades over why life
began so early on this planet, and then took so long to get interesting.
Some estimates indicate the earth was only a few tens of millions of
years old when the first simple organisms appeared. There was a little
evolution over the first billion years when single-celled organisms
morphed into bacteria, slimy algae and other simple kinfolk, but it was
still pretty dull.
It didn't get much better until nearly 600 million years ago when the
most dramatic period in the biological history of the planet erupted in
what has become known as the "Cambrian Explosion."
Those boring organisms from early earth evolved into forms of nearly
every plant and animal on the planet today in what has seemed like an
incredibly short period of time.
It seemed so fast, in fact, that Charles Darwin worried that it might
undermine his theory of evolution, thus giving birth to "Darwin's
dilemma."
Darwin thought evolution was a very slow process, proceeding in tiny
changes over many generations -- which is not well understood by some
people today. But if he was right, how could life have evolved so
quickly during the Cambrian era, advancing from simple forms to complex
plants and animals in the geological equivalent of the blinking of an
eye?
Of course, we know today that Darwin was right about evolution, but
wrong about the bleak fossilized record, which was incredibly thin when
he was around in the 1800s.
Excavations around the world have since uncovered fossils that show the
change was rapid, but not too rapid to be inconsistent with evolutionary
theory. It actually took millions of years.
But still, why it happened as quickly as it did is still highly
debatable. Something must have changed dramatically, and scientists
continue to chip away at that annoying conundrum. Two recent studies
published days apart may help clear the air.
A paper published last week in Science by Noah Planavsky of Yale
University and Christopher Reinhard of Georgia Institute of Technology,
based on ancient sediments from China, Australia, Canada and the United
States, suggests that scientists have long overestimated the amount of
oxygen in the earth's atmosphere in the pre-Cambrian era just before the
"explosion."
Many had thought the air was about 40 percent oxygen (around twice what
it is today) but oxidized chromium -- which is directly linked to oxygen
in the atmosphere -- in those sediments indicates the percentage was
only about one-10th of one percent.
No complex organism known today could survive in a world with that
little oxygen, so if this team is correct, the stage was not yet set for
rapid evolutionary processes. Something had to change before the
explosion could occur.
Meanwhile, other evidence published last week in the journal Geology
suggests that very dramatic changes driven by the tectonic breakup of
the so-called "supercontinents" of the pre-Cambrian era could have
caused an extraordinary leap in oxygen levels of both the ancient oceans
and the earth's atmosphere.
"I'm not claiming to have solved the Cambrian explosion," Ian Dalziel of
the University of Texas, Austin, said in a telephone interview. But he
has offered a scenario for how tectonic forces could have resulted in
oxygen enrichment and sea level rise.
The seas could have flooded the continents "which would have given rise
to lots of new ecological niches in shallow water for any creatures that
were around," he added.
Dalziel is not the first to suggest that rising sea levels could have
paved the way for an evolutionary explosion but his paper is believed to
be the first to integrate geological evidence from North and South
America, Africa, Australia and Antarctic.
Dalziel, who has studied this issue for 35 years, sees it happening something like this:
Movement of the tectonic plates underlying the ancient continents pushed
the tip of South America away from Antarctica, linking what is today
the Pacific and the Atlantic oceans. That movement would have extended
the length of mid-ocean ridges, which consists of subsea mountain ranges
where new magma rich in oxygen floods onto the ocean floor, releasing
nutrients and causing the sea level to rise. Dalziel likens that to
filling a bathtub with too much water and then plopping into the tub,
causing water to spill over the sides.
These two studies fit neatly together, suggesting that a world deprived
of oxygen could have changed relatively quickly into an incubator for
new life in shallow ponds spread across the continents and fed by waters
rich in nutrients. Perhaps that set the stage for the explosion.
The preparations would have taken many millions of years, since tectonic
plates move at about the same speed as the growth of human fingernails.
But if conditions were ripe enough the evolutionary rate may have
jumped to, say, five times the rate today.
And that's exactly what a team of researchers at the University of
Adelaide in Australia found when they measured the rate of evolution in
fossils dating back to 540 million years ago.
So the Cambrian explosion, long exploited by evolution opponents, may
not be so mysterious after all. These studies are certainly not the last
word on the subject, and much more research needs to be done to confirm
the findings, but Charles Darwin would undoubtedly be very encouraged.
