Scientists at the Massachusetts Institute of Technology say about 27 percent of all gene families that exist today were born between 3.3 billion and 2.8 billion years ago, ScienceNews.org reported Tuesday.

The surge of gene births, dubbed the Archean expansion, happened before some important changes occurred in Earth's early chemistry, including the appearance of large amounts of oxygen in the atmosphere, evolutionary biologists Eric Alm and Lawrence David say.

Fossils of organisms billions of years old are difficult to find but the researchers have found a rich molecular fossil bed billions of years old in the genetic blueprints of living organisms.

"Imprinted in the DNA of modern organisms is the history of these Precambrian events," Alm says.

The study may show how early organisms responded to and helped alter the planet's early chemistry, the researchers said -- noting that genes for using oxygen appeared at the end of the genetic expansion around 2.8 billion years ago, long before oxygen began accumulating in the atmosphere around 2.5 billion years ago.

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