Life on Earth had an easy birth, but a difficult childhood.
Scientists have recently painted a startling new picture of the Earth's early history, which, among other things, suggests that our planet once suffered through a super ice age from which it almost did not recover.
The new scenario fills an evolutionary gap that has puzzled researchers for decades. According to one researcher, our previous conception of the early history of Earth "has been stood on its head."
The discoveries are also helping scientists understand why it took more than 2 billion years for complex plants and animals to appear on the Earth, and why their appearance in the fossil record is so abrupt. And the new information is helping scientists appreciate the crucial role of carbon dioxide in regulating the Earth's climate.
This newly emerging picture is providing more and better information about the development of the global environment, especially at the crucial point in time when the atmosphere was changing from anaerobic, which could not support human life, to aerobic, which could.
Because the conditions at that time were unique, such a scenario is unlikely to occur on Earth again, but the information learned about that period will help researchers determine if life could have developed on other planets and in other solar systems, and how it might have evolved.
What researchers have learned is that, a little more than a billion years ago, the Earth's oceans were teeming with microscopic life--blue-green algae, phytoplankton, cyanobacteria and thousands of other single-celled species, the product of 2.5 billion years of slow evolution.
These microorganisms were proliferating so rapidly that they used up most of the carbon dioxide in the Earth's atmosphere, allowing the life-giving warmth normally trapped by atmospheric carbon dioxide to radiate into space--the opposite of the greenhouse effect that climatologists now fear is warming the planet.
Glaciers at Equator
Within 400 million years, the globe had cooled to the point where glaciers covered the Equator at sea level, and as many as two-thirds of all the species on the planet had been snuffed out by cold. This "snowball Earth," researchers say, came perilously close to being forever entombed in ice, and was rescued only by the release of more carbon dioxide from the Earth's mantle by volcanoes.
That super ice age was much longer and much colder and had greater impact on living organisms than normal ice ages. Normal ice ages, which are caused by natural variations in the Earth's orbit, persist about 100,000 years, with 10,000- to 20,000-year intervals of warmth between them. The super ice age was superimposed on top of this normal pattern of ice ages and lasted for tens of millions of years.
"It looks like the world's ecological system collapsed slowly," paleobiologist J. William Schopf of UCLA said. "That sort of thing has never been observed anytime before or since. It's completely different from the sort of extinctions one thinks about in the rest of the geological record."
But the Earth made a miraculous recovery and, as the planet slowly warmed, new forms of life began to appear--multicelled organisms with hard shells or internal bone structures unlike anything that preceded them in the fossil record.
In a sense, geologist Joseph Kirschvink of Caltech said, the slow creep of glaciers to the Equator had the effect of "scraping off the planet down to the bone, so that life suddenly had a new environment to inhabit." And once those new environmental niches were established, life proliferated and evolved rapidly, progressing from simple multicelled organisms all the way to humans in only 600 million years, less than 15% of the Earth's history.
The result of this remarkable picture of the Earth's adolescence, geochemist John M. Hayes of Indiana University said, is that "our approach to the Earth's history has been stood on its head." Where researchers once thought that levels of carbon dioxide and oxygen in the atmosphere had changed only slowly and by only small amounts, he said, the new evidence has suggested much greater changes, accompanied by large changes in the nature of life.
"It's exciting times," Hayes said of the current research. "There are really fundamental ideas being changed and concepts being developed that are of the first magnitude in terms of their significance."
10-Country Research Project
The new discoveries came from an unusual collaboration of more than 50 researchers from 10 countries. They have been drawn together for the past five years in an unprecedented research project under the aegis of Schopf, head of UCLA's Center for the Study of Evolution and the Origin of Life.
The group, called the Precambrian Paleobiologic Research Group--Proterozoic, or PPRG--P, has carried out an extensive series of biological and geological field trips and collected more than 1,800 rock specimens for laboratory analysis--an unprecedented number of specimens for this period of history.
The Precambrian Era, the period from the Earth's formation about 4.5 billion years ago until roughly 570 million years ago, has long been a source of puzzlement because the fossil record--the mineralized remains of plants and animals--appeared to begin abruptly roughly 570 million years ago. The oldest fossils that had been discovered, until about 40 years ago, were of trilobites, hard-shelled, segmented forerunners of modern insects, spiders and crabs.
In the 1950s, however, the late Stanley Tyler of the University of Wisconsin and Elso S. Barghoorn of Harvard University showed that unusual microscopic structures found in quartzite rock from the northern shores of Lake Superior were actually fossils of algae dating from 2.1 billion years ago.
More recently, Schopf and others have found still older fossils, the most ancient being 3.5-billion-year-old fossils of slimy green bacteria that colonized muddy ponds in western Australia.
Despite their great age, the fossils Schopf discovered were not as simple as might have been expected. In fact, they formed filaments and colonies that did not seem much different from those formed by similar algae and bacteria today. Schopf and others concluded that life must have begun fairly soon after the Earth was formed, perhaps within the first half-billion years, for evolution to have progressed that far.
But in solving one mystery, Schopf had created another. If life could begin and evolve into sophisticated bacteria so easily in the first few hundred million years of the planet's history, why were another 2 billion years required before the first multicelled organisms appeared?
"We're talking about a vast amount of time," said micropaleontologist Carl V. Mendolson of Beloit College in Beloit, Wis. "We're trying to read this book of life based on very few pages."
Based on the work of the PPRG-P, though, researchers are beginning to pull together the first comprehensive picture of what the Earth must have been like during the Proterozoic Period, that portion of the Precambrian Era ranging from 2.5 billion years ago up to 570 million years ago. And that picture "strains some of the evolutionary scenarios that have been widely accepted," geophysicist J. John Sepkoski Jr. of the University of Chicago said.
Paleontologists have long believed that the Proterozoic was characterized by continuous and slow evolution, a process that led naturally to the evolution of the complex, multicellular species. But the new evidence indicates, Sepkoski said, "that is not at all correct. Early on in the Earth's history, a reasonably stable (biological) community was put together and it really didn't change through the Proterozoic."
Only when the climate underwent a near-catastrophic upheaval at the end of the Proterozoic did the evolutionary pace begin to accelerate again. "There wasn't an evolutionary march toward greater and greater complexity, toward modernity, but a record of jumps and starts," Sepkoski said.
The key evidence supporting this new scenario of the Earth's evolution lay in the residual magnetism of 1-billion-year-old rocks studied by George E. Williams of the Commonwealth Scientific and Industrial Research Organization and Brian J. J. Embleton of Macquarie University, both in North Ryde, Australia.
That magnetism indicates that Australia was on the Equator a billion years ago. Evidence from sediments formed at that time also indicates the continent was covered by glaciers. If Australia, on the Equator, was covered by glaciers, then the whole world must have been, because the ice would have spread from the poles.
Why did the glaciers form? Scientists blame the loss of carbon dioxide from the Earth's atmosphere.
Carbon Not Released
About 1 billion years ago, the Earth's oceans were filled with microscopic life that extracted carbon dioxide from the air and converted it into sugar molecules that became part of their cellular structures.
As they died naturally, the microorganisms drifted to the bottom of seas and were buried in sediments, so that the carbon they contained could not be released back into the atmosphere.
Today this process is comparatively rare because the bulk of the microorganisms are eaten by animals and their carbon recycled through the food chain. Most researchers agree that the circumstances that caused global glaciation were unique and that the event could not happen again.
"Organic carbon was being buried at rates as high as anything we've seen in the last billion years," said Andrew Knoll of Harvard, whose study of sediments has been crucial in documenting the changes in carbon dioxide levels.
As new generations of microorganisms removed more and more carbon dioxide from the atmosphere, temperatures slowly grew colder and glaciers crept toward the Equator from both poles, eventually enveloping the whole planet.
As the climate cooled, whole species began dying off, Schopf said. According to the fossil record, he added, "at least half, maybe two-thirds of the things that were alive didn't make it after about 600 million years ago."
Most Life Died
The only thing that saved the Earth from completely freezing over, apparently, was the fact that most life had died off. Living organisms were therefore no longer removing much carbon dioxide from the atmosphere, and the carbon dioxide released by volcanoes began accumulating, gradually warming the planet.
In a remarkable coincidence, researchers said, at just about this time oxygen began accumulating in the atmosphere. Living organisms thus confronted an ecology that had been stripped of most of the previous life forms and had an abundance of oxygen to meet their needs.
Climatologists know that the first fossils of multicellular animals are found in sediments lying immediately above those deposited during the glacial period--and they are found all over the world. "Some external event triggered the ability of organisms to mineralize and grow larger . . . and one very good possibility is the oxygenation of the atmosphere," biologist Bruce Runnegar of UCLA said.
Because of all the tectonic, atmospheric and environmental shifts, said Knoll, "we have to conclude that the world changed in a fairly fundamental way."
A Super Ice Age Earth was in danger of becoming totally encased in ice forever Researchers trying to solve they mystrey of why complex creatures suddenly appeared on the Earthsome 570 million years ago have pieced together a stratling new picture of an Earth that almost became a massive ball of ice. As carbon dioxide was removed from the atmosphere by natural processes, the Earth grew colder until glaciers covered even the equator. As many as two-thirds of all species were killed off in the freeze. Earth 800 million years ago Earth's land masses were concentrated into two large supercontinents, centered on the equator. When the amount of carbon dioxide in the air reached its lowest level, Earth was encased in ice. Earth 600 million years ago to now The extinction of so many species, combined with growing concentrations of oxygen in the air, allowed new forms of organisms to appear and begin to spread over the planet in a new stage in theevolution of life on Earth. Life 3 billion to 800 million years ago Life in the oceans consisted of large groups of one-celled micoorganisms. Oceans z3 billion to 800 million years ago One-celled organisms proliferated, converting carbon dioxide into cellular materials. As they died, they sank to the bottom and were buried. Glaciers 1 billion to 800 million years ago As carbon dioxide was removed from the atmosphere, the Earth got colder and glaciers at the poles began to expand and move toward the Equator. Over time the glaciers ended up covering almostthe entire planet including the oceans. Volcanoes 800 million to 600 million years ago Earth was saved from a permanent deep freeze when volcanoes erupted and released carbon dioxide into the atmosphere. Thus began a process to replace the carbon dioxide taken out by one-celledorganisms.