All life today descended from a microbe

Life forms were pretty underwhelming four billion years ago. Primitive microbes dwelled in iron-rich hot springs. They probably didn’t look like much or do a whole lot – they lived off hydrogen, carbon dioxide and nitrogen gases bubbling through their warm, watery home.

But they’re your ancestors – and the last common ancestor of all life today, according to a German genetic study.

Researchers at the University of Düsseldorf, wanting to unpick what the last universal common ancestor was like, sorted through 6.1 million genes found in single-celled organisms today.

In a study published in Nature Microbiology, they found 355 protein groups that were likely retained from our ancestor microbe, from which the bacteria and archaea split four billion years ago.

And as genomes are usually well adapted to their environment, writes James McInerney, a biologist at the University of Manchester in the UK, knowing parts of the ancient microbe’s genetic make-up can also give clues to its habitat.

Tracing our genetic heritage through millions – let alone billions – of years is a tricky business. Genes aren’t only passed down generations from parent to offspring, but are muddied by what’s called horizontal gene transfer. 

For instance, one species of bacteria might lop off some DNA and transfer it into another species. The new genetic material can, and often does, function perfectly well in its host.

This means if a number of bacteria species have the same gene, it doesn’t necessarily mean that they inherited it from a common ancestor. It could have just as easily been horizontally transferred.

So Madeline Weiss, Filipa Sousa and colleagues were brutal in narrowing down potential genes that were probably present four billion years ago, when bacteria and archaea are thought to have diverged from a common ancestor.

(Eukaryotes – organisms with cells containing membrane-bound nucleus and organelles, such as those that make us – came later.)

An algorithm sorted than 6.1 million protein coding genes from 1,847 bacterial and 134 archaeal genomes, downloaded from the open access National Center for Biotechnology Information Reference Sequence database, into 286,514 protein clusters.

Of these clusters, only 355 were present in at least two bacterial species and two archaea.

From this genetic information, the team reconstructed its lifestyle. It – like some bacteria and archaea today – used that’s known as the Wood–Ljungdahl pathway, which nicks an electron from a hydrogen gas molecule and slots it in a carbon dioxide molecule.

To get its hydrogen, it must have lived in a hydrothermal vent – the only geological source of hydrogen gas on Earth at the time. In a process called serpentinisation, positively charged iron in the crust splits water into its constituent oxygen and hydrogen parts, so the water must have been iron-rich.

Of microbes living today, Weiss, Sousa and colleagues write it was most like clostridia bacteria and methane-producing archaea.

All this was happening during a particularly turbulent time in our solar system called the Late Heavy Bombardment. As its name suggests, objects circling the sun were pummelled by other objects, such as asteroids, comets and protoplanets (the beginnings of planets).

The moon, for instance, is thought to be the result of a protoplanet collision.

This bombardment, McInerney writes, probably meant Earth’s oceans were heated and vapourised on a regular basis, probably leading “to bottlenecks in the diversity of life at the time, meaning only the hyperthermophiles [organisms that live in very hot water] survived”.

So while our microbial ancestor was not the first form of life, it was in the right place at the right time to survive the planet’s celestial beating.