Researchers have discovered an early branch in the family tree of acetogens, possibly one of the earliest life species on Earth. Due to differences in an important biochemical process in the single-celled organisms, one branch more closely resembles a precursor of bacteria, and the other more closely resembles a precursor of another important class of microorganisms: the archaea. The American researchers published their studies Monday in PNAS.
Acetogens get their name from the fact that they contain acetate (CH3CO2–) produce. Acetate is one of the simplest possible organic substances – the molecules of which life is built. They live on the seabed near hot springs, where they feed on simple molecules that spray up from the Earth’s core, such as carbon dioxide (CO2) and hydrogen (H2). That makes them special: they are not dependent on substances produced by other micro-organisms in an ecosystem.
Lifeless Planet
That means they could have originated first in a lifeless planet. And because they can create simple organic substances, they may have fueled the further evolution of life on Earth. Those who study them may well be looking at primordial evolution.
That’s what the researchers did, in a rocky place that looks a lot like what the early Earth must have looked like. They visited the geologically interesting Samail Ophiolite, a rock formation in the Hajar Mountains of the state of Oman. This site consists of a piece of oceanic rock from beneath the Earth’s crust, which has been brought up by the movement of tectonic plates. The geological processes from which the earliest microorganisms lived on the seabed are still active.
The researchers pumped water up from beneath the rock formation, and found multiple populations of acetogens. They analyzed the DNA of the micro-organisms and identified two species. Both have a clear preference for their habitat: one is mainly found in very deep and alkaline waters (the opposite of acidic waters), the other lives a bit higher and in more neutral water. They are very close together in the evolutionary tree, but have just taken a different evolutionary turn.
More complex organisms
The difference between the two is mainly in the way the acetogens make acetate. The biochemical reaction process of one species is very similar to that of early bacteria, the other is more like that of so-called ‘archaea’ – both types of microorganisms are the first two types of life on Earth. It then took more than a billion years for the third species to emerge: more complex, multicellular organisms.
“This excellent study makes a great contribution to the theory of the origin of life,” judges Michael Russell. He is a geochemist at the University of Turin and the founder of the idea that life may have originated around the rock as in the Hajar Mountains. “It provides evidence that the very small population of early microbes grew, survived and shows how evolution has progressed.”
“Little is known about contemporary acetogens, while such organisms have long been believed to live in these environments and to be among the earliest species of life,” said Daniel Colman, a microbiologist at Montana State University and author of the study. “We now show that longstanding hypotheses about acetogens are consistent with real-world examples.”