December 20, 2010

3 Billion-year-old Genomic Fossil Deciphered

Life on Earth began to flourish about 3 billion years ago, possibly when primitive forms developed efficient ways to harness energy from the Sun's light, according to a new study published in the journal Nature on Sunday.

Scientists at the Massachusetts Institute of Technology (MIT) built a "genomic fossil" -- a mathematical model that took 1,000 key genes that exist today -- and calculated how they evolved from the very distant past.

The collective genome of all life expanded massively between 3.3 and 2.8 billion years ago, and during this time roughly 27 percent of all presently existing gene families came into existence, the study suggests.

Eric Alm and Lawrence David, investigators working on the project, said the great surge probably began with the advent of a biochemical process called modern electron transport.

This is a key biological function, they explain, involving the movement of electrons within the membranes of cells. It is central to plants and to some microbes, allowing them to harvest energy from the Sun through photosynthesis and to breathe oxygen.

Alm and David said the big change, dubbed the Archean expansion, was followed some half billion years later by a phenomenon known as the Great Oxidation Event, when Earth's atmosphere became increasingly flooded with oxygen.

The Great Oxidation Event is possibly the biggest species turnover in the history of our planet, as primitive or microbial forms of life that were non-oxygen-breathing organisms died out and were replaced by bigger, smarter aerobic forms.

"Our results can't say if the development of electron transport directly caused the Archean Expansion," David told AFP. "Nonetheless, we can speculate that having access to a much larger energy budget enabled the biosphere to host larger and more complex microbial ecosystems."

Most early fossils date back to the Cambrian Explosion, some 588 million years ago.

But pre-Cambrian lifeforms were soft-bodied and, with some rare exceptions, did not leave a fossil imprint. Although, they did leave a trail of abundant DNA, which explains the efforts to recreate the "genomic fossil" by computer.

"What is really remarkable about these findings is that they prove that the histories of very ancient events are recorded in the shared DNA of living organisms," said Alm. "Now that we are beginning to understand how to decode that history, I have hope that we can reconstruct some of the earliest events in the evolution of life in great detail."


Image 2: The figure shows the evolution of gene families in ancient genomes across the Tree of Life. The sizes of the little pie charts scale with the number of evolutionary events in lineages, slices indicate event types: gene birth (red), duplication (blue), horizontal gene transfer (green), and loss (yellow). The Archean Expansion period (3.33 to 2.85 billion years ago) is highlighted in green. Credit: Lawrence David


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