Earth was a very different place 4 billion years ago. Its air lacked oxygen, its surface was pummeled by space rocks, and its seawater sometimes boiled. Still, it was already home to your ancestors, who lived among volcanoes on the ocean floor.

Those early Earthlings, a new study suggests, were the last common universal ancestor of life on Earth, a lofty title abbreviated as LUCA.

Scientists have been wondering about LUCA for a long time, hoping its identity might offer clues about how life on Earth began. This mysterious creature gave rise to all three "domains" of life we know today — archaea, bacteria and eukaryotes — so its descendants include everything from E. coli to elephants.

And now, thanks to some deep genetic sleuthing, a team of researchers from Germany has pieced together a remarkably detailed picture of what LUCA's life was probably like. Published this week in the journal Nature Microbiology, their study suggests LUCA was a single-celled, heat-loving, hydrogen-eating microbe that lived without oxygen and needed certain kinds of metals to survive.

tubeworms on hydrothermal vents Hydrothermal vents now support a wide variety of life, like these tubeworms, anemones and mussels spotted 1.6 miles (2.6 kilometers) deep in the eastern Pacific Ocean. (Photo: NOAA Ocean Explorer/Flickr)

Life near hydrothermal vents

Based on these and other traits, scientists say LUCA most likely lived among deep-sea hydrothermal vents — fissures in Earth's surface (including the ocean floor) that release geothermally heated water, typically near volcanoes. This kind of life was unknown until 1977, when scientists were amazed to find diverse arrays of weird organisms thriving around hydrothermal vents off the Galapagos Islands. Instead of getting energy from sunlight, these dark ecosystems rely on chemical processes triggered by seawater interacting with magma from underwater volcanoes.

We've since learned a lot about hydrothermal-vent ecosystems, from bizarre tubeworms and limpets to chemosynthetic archaea and bacteria at the base of the food web. Astronomers even suspect similar vents exist on other worlds, like Jupiter's moon Europa, raising the possibility they could harbor alien life.

Here on Earth, some scientists also speculate that early life evolved around hydrothermal vents on the ocean floor. That's still debated, though, with many experts arguing the conditions for abiogenesis were more favorable on land. The new study may not settle that debate, but it does provide an intriguing glimpse of life 4 billion years ago — and of the tiny beings to which we all owe our existence.

methanogenic archaea Methanogens are a type of archaea 'whose modern lifestyles resemble that of LUCA,' the researchers write. (Photo: NASA)

How to look for LUCA

Previous studies have shed some light on LUCA, Robert Service notes in Science Magazine: Like modern cells, LUCA built proteins, stored genetic data in DNA and used molecules known as adenosine triphosphate (ATP) to store energy.

Yet our image of LUCA has remained hazy, partly because microbes don't just pass genes to their offspring; they also share genes with other microbes, a process known as horizontal gene transfer. So when two modern microbes both have certain genes, it can be difficult for scientists to know if that really points to a common ancestor.

Difficult, but not impossible. Led by William Martin, an evolutionary biologist at Heinrich Heine University in Dusseldorf, Germany, the new study tried a slightly different tactic to figure out which genes were inherited. Instead of hunting genes shared by one bacterium and one archaeon, the study's authors looked for genes shared by two species of each. That turned up 6.1 million protein-coding genes, which fall into more than 286,000 gene families. Of those, only 355 were distributed widely enough in modern life to suggest they're relics of LUCA.

"Because these proteins are not universally distributed," the researchers add, "they can shed light on LUCA's physiology." Namely, these protein-coding genes reveal LUCA was an extremophile, or an organism that thrives in extreme environments. It was anaerobic and thermophilic — meaning it inhabited an oxygen-free habitat that was very hot — and it fed on hydrogen gas. It also used something known as the "Wood–Ljungdahl pathway," which lets some modern microbes convert carbon dioxide into organic compounds and use hydrogen as an electron donor.

snowblower hydrothermal vent, Axial Seamount A 'snowblower' hydrothermal vent, located off the Oregon coast, spews hot water and white chunks of bacteria that bloom in its chemical-rich water. (Photo: Bill Chadwick/Oregon State University/Woods Hole Oceanographic Institution/Flickr)

Martin and his co-authors identify two modern microbes with lifestyles resembling LUCA's: clostridia, a class of anaerobic bacteria, and methanogens, a group of hydrogen-eating, methane-producing archaea. They may offer us a living hint not just of what LUCA was like, the researchers say, but possibly even earlier ancestors.

"The data support the theory of an autotrophic origin of life involving the Wood–Ljungdahl pathway in a hydrothermal setting," they write, referring to primitive aspects of LUCA's biology that could indicate an early role in the rise of life.

That conclusion is less widely accepted, Nicholas Wade reports in the New York Times, as other biologists argue life likely started in shallower surface water, or that it could have arisen elsewhere before being relegated to the deep ocean.

We may never know exactly how or where life began, but the question is too compelling for us to stop trying. Humans are curious and dogged by nature, traits that have served our species well. And while we're very different from LUCA now, the ongoing legacy of this tiny ancestor suggests tenacity runs in the family.
