new discoveries in evolution

Earthly organisms developed in two sets of fits and starts: study

WASHINGTON (AFP) – Life on Earth went from single microscopic cells to blue whales and giant sequoias in 3.5 billion years in two distinct bursts tied to the planet's geological evolution, according to a new study.

The study, published in the Proceedings of the National Academy of Sciences, goes against the common hypothesis that life slowly evolved from a single-celled organism to complex multi-celled organisms.

"We were surprised to observe that nearly all of the increase in size occurred in two distinct time-intervals," said Michal Kowalewski, a co-author of the study and professor of geosciences at Virginia Tech.

"And what is more, those intervals followed two major oxygenation events."

In analyzing fossil records, the study found that the two size jumps were from bacteria to eukaryotic cells, and from unicellular to multi-cellular organisms.

Only bacteria-like fossils were found in the first 1.5 billion years of recorded life on Earth, and they did not grow until more complex organisms developed around 2 billion years ago after the appearance of eukaryotic cells.

Over three billion years ago, primitive bacteria "invented" photosynthesis, nourishing themselves from the sun's energy and carbon dioxide. They thrived in low-oxygen environments.

But the oxygen they released into the atmosphere allowed for more complex cellular structures to develop, and thus the eukaryotic cell was born. During about two hundred million years, organisms grew from cells invisible to the naked eye to organisms about the size of a thumbnail.

"In a way, thus, an increase in size and complexity was a consequence of geobiological interactions between life and earth. Life itself enabled life to become more complex," Kowalewski said.

It was not until another big increase in oxygen levels -- reaching as much as 10 percent of current oxygen concentration -- about 540 million years ago, that life developed into multi-cellular, tissue-forming organisms.

"What is really interesting is that each of these 'steps' correlate with a time in life's history where there is innovation in the complexity of life, the first one being the eukaroytic cell and the second is the multicellularity of life," said Virginia Tech researcher Jennifer Stempien, another study co-author.

Through the 3.5 billion years of documented history of life on Earth, the scientists found that organisms' maximum body size increased by 16 times.

Today, marine animals like blue whales or vascular plants like the giant sequoia -- the largest known form of life -- can reach body sizes greater than the biggest dinosaurs.

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