杏吧原创

The shared ancestor of every living organism on the planet lived at least 3.9 billion years ago 鈥� adding weight to theories that life started 100 million years earlier than we thought, at a time when the Earth was still being pummelled by meteorites.

That鈥檚 according to a study that has combined genetic and fossil evidence to build a timeline of crucial shifts in the early evolution of life. The study also sheds new light on the birth of complex cells, which today make up all animals and plants.

For the first few billion years of Earth鈥檚 history, the only life present were single-celled microorganisms. Unlike the large animals and plants that arose in the last 600 million years, like trilobites and dinosaurs, these microbes left few fossils 鈥� so understanding life鈥檚 early history has been tricky.

A huge family tree

To find out more, at the University of Bristol and her colleagues combined two sources of evidence. They compared the sequences of 29 genes across 102 species, to build a family tree that showed how they were all related, and the order in which new groups split away from their relatives.

The team then added some dates to this 鈥減hylogenetic鈥� tree, taken from the geological record, enabling them to estimate when the various groups evolved and split from each other.

For instance, they knew that life could not be any older than 4.52 billion years 鈥� because that is when a rock the size of Mars slammed into the Earth, forming the Moon. The impact was so severe that Earth鈥檚 entire surface melted. 鈥淣othing could have survived it,鈥� says team member .

The result is a timeline of the first three billion years of life, from the ancestor of all modern life to the first complex animals. 鈥淲e can go very deep in time, which we never thought was possible,鈥� says Pisani.

Everyone鈥檚 grandmother

鈥淚 think they鈥檝e really made the most sincere and honest effort yet to actually get a coherent picture of molecular phylogeny and the history of life,鈥� says of the University of D眉sseldorf in Germany.

Two main findings emerge from the new timeline of life. The first is that the most recent organism that all existing life is related to 鈥� known as the last universal common ancestor (LUCA) 鈥� lived at least 3.9 billion years ago.

It is feasible that it could have existed this early in our planet鈥檚 4.5 billion-year history. Some have suggested that traces of carbon in 4.1-billion-year-old rocks are . Similar traces were found in in 2017. And another 2017 study claimed to have found fossil single-celled organisms from 3.77 billion years ago. But all these findings are disputed.

Nevertheless, at the Massachusetts Institute of Technology, says the team鈥檚 date for LUCA is reasonable.

This means that life likely formed on Earth quickly, within 600 million years of the Moon-forming impact, when the planet was still being battered by big meteorites.

It鈥檚 unclear what, if anything, this might mean for the various competing hypotheses on where and how life began. Researchers like Martin believe life began in alkaline vents on the sea floor, something Martin鈥檚 own LUCA studies have supported. It鈥檚 an idea Pisani is sympathetic to. 鈥淭hese [deep-sea vents] would be sheltered in a condition of heavy bombardment,鈥� he points out.

In contrast, of the MRC Laboratory of Molecular Biology in Cambridge, UK has argued that a lake in a meteorite crater would offer the ideal chemistry to form organic molecules and simple cells. A LUCA that lived during a bombardment period fits that scenario.

The first inklings of you

However LUCA formed, it gave rise to two groups of single-celled microorganisms called bacteria and archaea. This split happened at least 3.4 billion years ago, the study finds.

Later on, more complex cells evolved 鈥� one of the most important events in the entire evolution of life. Compared to bacteria and archaea, the cells of 鈥�eukaryotes鈥� are larger, have a nucleus that contains their genetic material, membrane-bound compartments, and tiny sausage-shaped structures called mitochondria, which supply them with energy.

All animals, plants and fungi are eukaryotes, and if these cells had not evolved, humans could never have existed. The new timeline suggests that these cells first appeared between 1.21 and 1.84 billion years ago.

It had been thought that eukaryotes evolved in response to rising oxygen levels, but this finding suggests this might not have been the case, as the first atmospheric oxygen appeared much earlier 鈥� 2.4 billion years ago. However, it does not rule out a link, as at first there was little oxygen and levels later rose in fits and starts.

The timing of the eukaryotes鈥� origin is curious, because it is almost exactly when a group of bacteria called alphaproteobacteria evolved. The mitochondria inside eukaryotic cells were once free-living alphaproteobacteria, which were somehow swallowed by an archaean that then gave rise to the first eukaryotes.

Perfect power-house

The implication, says Pisani, is that acquiring mitochondria was the most crucial step in the evolution of eukaryotes. 鈥淚t is consistent with the view that the symbiosis with the mitochondrion was the emergence of something different to what existed before,鈥� he says. This spurred a massive evolution of diversity among eukaryotic cells, he says.

Martin and his colleagues have made a similar argument on physical grounds. 鈥淎ll of the cells that are complex have or had mitochondria,鈥� he says. 鈥淭hat鈥檚 not pure coincidence, because all these evolutionary innovations that separate the eukaryotes from [bacteria and archaea], cost energy.鈥� The idea is that the first eukaryotes could only evolve large, complex cells once they had mitochondria to super-charge them.

However, Fournier suspects the timings might be off. He says mitochondria are quite different from their alphaproteobacteria ancestors, 鈥渟o they should be substantially younger鈥�.