In the last few years, DNA analysis has allowed researchers to redraw the tree of life in incredible detail, but there’s always been a question mark at the base of the tree. While it’s unlikely that researchers will ever find the exact species that started it all, they recently came up with a pretty good description of LUCA, the Last Universal Common Ancestor of all of Earth's creatures, sometimes referred to as microbial Eve.
Life as we know it is currently divided into six kingdoms: plants, animals, fungus, protists, eubacteria and archaebacteria. The first four belong to the a domain known as eukaryotes, sporting cells with distinct nuclei. The other two kingdoms, eubacteria and archaebacteria are single-celled organisms without a distinct nucleus. All of them evolved from a single-celled ancestor that lived about 4 billion years ago when Earth was celestial baby.
After all those billions of years of change, LUCA’s fingerprints are still visible in the genes of modern organisms. That’s why William Martin, an evolutionary biologist at Heinrich Heine University in Düsseldorf, Germany, set out to study LUCA's trail in the genes of bacteria and archaea, the two groups researchers believe became eukaryotes.
Tracking genes in bacteria is particularly difficult because they can swap genetic material, making it hard to discern whether the single-celled organisms received a gene from an ancestor or picked it up from another species along the evolutionary road, reports Robert F. Service at Science. So Martin and his team decided to search for genes shared by at least two species of modern bacteria and two archaea, an indicator that the gene was likely inherited and not an evolutionary hitchhiker.
The researchers combed through DNA databanks, analyzing the genomes of 2,000 modern microbes sequenced over the last two decades. From six million total genes, they found 355 gene families that were widespread among the microbes, which means they were likely to be genes LUCA passed down. They published their results in Nature Microbiology.
LUCA’s genes are those of an extremophile organism that likely lived in an area where seawater and magma meet on the ocean floor, known as hydrothermal vents, reports Nicholas Wade at The New York Times. Similar creatures still haunt these environments among the toxic plumes of sulfides and metals. And many researchers already believe this is where life first began.
“I was flabbergasted at the result, I couldn’t believe it,” Martin tells Michael Le Page at New Scientist. “It’s spot on with regard to the hydrothermal vent theory.”
The genes show that LUCA lived in habitat with no oxygen, Service writes. It also fed on hydrogen gas, meaning it was likely an organism that lived near super-heated volcanic vents where hydrogen gas was likely produced. LUCA’s lifestyle is similar to two types of microbes that researchers have uncovered, the anaerobic bacteria in the genus clostridium and the hydrogen gobbling archaea in the methanogens group, James Lake, an evolutionary biologist at UCLA tells Service
But not everyone is convinced that the hydrogen gobbling vent-dweller Martin uncovered is really LUCA. John Sutherland of the University of Cambridge in England, whose research suggests the origins of life began on land and not deep in the ocean, tells Wade that life could have developed elsewhere and then been shoved down into places like hydrothermal vents during global disasters like the Late Heavy Bombardment, a catastrophic period in Earth’s history between 4 billion and 3.8 billion years ago in which the planet was reshaped by a shower of asteroids and comets.
In fact, he argues that basic chemistry shows life likely originated in pools of water on land, Darwin’s “warm little ponds.” Ultraviolet light from the sun, which does not reach down to hydrothermal vents, he argues, is a key element in that chemistry.
More research is necessary for scientists to unravel the twisting branches of the tree of life and to determine if Martin’s LUCA is a super-great aunt or the microbial Eve.