About 4 billion years ago, molecules began to make copies of themselves, an event that marked the beginning of life on Earth. A few hundred million years later, primitive organisms began to split into the different branches that make up the tree of life. In between those two seminal events, some of the greatest innovations in existence emerged: the cell, the genetic code and an energy system to fuel it all. All three of these are essential to life as we know it, yet scientists know disappointingly little about how any of these remarkable biological innovations came about.
“It’s very hard to infer even the relative ordering of evolutionary events before the last common ancestor,” said Greg Fournier, a geobiologist at the Massachusetts Institute of Technology. Cells may have appeared before energy metabolism, or perhaps it was the other way around. Without fossils or DNA preserved from organisms living during this period, scientists have had little data to work from.
Fournier is leading an attempt to reconstruct the history of life in those evolutionary dark ages — the hundreds of millions of years between the time when life first emerged and when it split into what would become the endless tangle of existence.
He is using genomic data from living organisms to infer the DNA sequence of ancient genes as part of a growing field known as paleogenomics. In research published online in March in the Journal of Molecular Evolution, Fournier showed that the last chemical letter added to the code was a molecule called tryptophan — an amino acid most famous for its presence in turkey dinners. The work supports the idea that the genetic code evolved gradually. Using similar methods, he hopes to decipher the temporal order of more of the code — determining when each letter was added to the genetic alphabet — and to date key events in the origins of life, such as the emergence of cells. Via Paleogenomics Reconstructs Ancient Proteins