Life has been around for billions of years, but it's only about 580 million years ago that organisms start leaving behind fossils. But our modern genes provide a window into the past, revealing how life developed billions of years ago.
The genomes of modern organisms are all inherited from their ancestral counterparts, which basically means our genomes preserve genetic "fossils" of our ancient ancestors. That was the theory put forward by MIT computational biologists, who hoped to take all available genomic data - thousands of genes from over 100 modern genomes - and figure out how genes evolved in ancient microbes.
Of course, our genomes aren't a perfect record of our ancestors'. Gene families can be inherited from one generation to the next, yes, but they can also be born new, lost along the way, duplicated in the same genome, and even lost. But with all those genomes to work with and some nifty mathematical models, the researchers were able to trace the evolutionary history of our genomes going back billions of years.
They discovered that 27 percent of all living gene families came into existence during a single period, between 3.3 and 2.8 billion years ago. They've called this period the Archean Expansion, and it's one of the most productive periods in the entire history of life...but what caused it?
Many of the Archean genes deal with oxygen, so the researchers initially thought the genes might have been created during the Great Oxidation Event. This was the period about 2.5 billion years ago when oxygen started to accumulate in the atmosphere, killing off most of the anaerobic lifeforms and paving the way for aerobic life.
But, as you might have noticed, the numbers don't quite work, as the Great Oxidation Event occurred a little too late to be responsible for all these new genes. Instead, the researchers suspect they've discovered the birth of electron transport. This biochemical process moves electrons within the cellular membrane, allowing the cells to breathe oxygen and helping make oxygenic photosynthesis possible. It's that sort of photosynthesis which, unsurprisingly, paved the way for the Great Oxidation Event.
Researcher Lawrence David explains how electron transport could have created so many new gene families:
"Our results can't say if the development of electron transport directly caused the Archean Expansion. "Nonetheless, we can speculate that having access to a much larger energy budget enabled the biosphere to host larger and more complex microbial ecosystems."
His colleague Eric Alm explains why this finding is important, and what they hope to achieve:
"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. And 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."
[Nature; image is artist's conception of meteorite bombardment billions of years ago.]