Scientists Think They’ve Found the Ancient Ancestor of All Life on Earth
Meet LUCA, the shared relative of you and me and everything else.
Underwater chimney vents near the Northwest Eifuku volcano, Japan. Image: Wikimedia Commons
As early as 3.8 billion years ago, life arose from the toxic waters of Earth's primordial slurry. Our ancient ancestor wasn't large, or complex, or even multicellular. As far as scientists know, it was probably just a single-celled prokaryote, resembling bacteria more than anything in the animal kingdom.
Yet, beyond this rough genealogy, little is understood about our first relative's environment, behavior, and needs. LUCA, which stands for our hypothetical "last universal common ancestor," left no fossil evidence of its existence. In fact, its only lasting memorial is a faint imprint on the genes of every living creature today.
In a study published this week in Nature Microbiology, scientists at Heinrich Heine University in Düsseldorf, Germany, used this far-flung DNA evidence to produce astounding new discoveries about LUCA.
For the very first time, we have what's believed to be a portrait of this early forebearer, and evidence that life might have emerged around hydrothermal vents. If proven, these findings would topple Charles Darwin's longstanding theory that organic life was conceived "in some warm little pond," and not in the depths of a Precambrian ocean.
The team's findings were based on big data analysis of 6.1 million protein coding genes. By parsing the genetic lineage of bacteria and archaea, the two oldest domains of cellular life, they were able to identify 355 shared genes that probably originated from LUCA, and persist in modern prokaryotes.
Previously, it was thought that LUCA gave rise to all bacteria, archaea, and eukaryotes. This three-domain, "universal tree of life" was widely accepted and propagated until recent genomic investigations led scientists to believe that eukaryotes—plants, animals, fungi, and other organisms whose cells contain membrane-enclosed nuclei—actually emerged from prokaryotes, like bacteria and archaea.
So, with eukaryotes surfacing much later down the evolutionary timescale, the study's authors honed in on the ancestral genes of the other two domains. Using several decades of genetic information, the team isolated LUCA's presence in a combined 286,514 protein clusters, or the essential building blocks to life. If genes were present in both bacteria and archaea, their DNA code—letters A, C, G, and T—would be sorted and arranged into a phylogenetic tree, eventually leading back to a single, unique organism.
From this mountain of information, the 355 protein families traced back to LUCA revealed a profile of the creature that lived some four billion years ago. According to the study, LUCA lived around hydrothermal plumes, and thrived in an environment devoid of oxygen. It was "autotrophic," or self-sustaining, and likely derived energy from its carbon dioxide and hydrogen rich surroundings. LUCA possessed a particular enzyme that would have made it possible to live in a hot, and possibly volcanic, environment. This same enzyme also enabled our ancestor to potentially synthesize the metals and minerals that exist near underwater vents, similar to the survival tactics used by today's extant deep sea "thermophiles."
Although this description of LUCA fits the hellish environment of Earth at that time, some scientists are skeptical of the report. In interviews with the New York Times, critics proposed that LUCA was actually a highly-evolved iteration of the first living organism. Others remarked that it was only "half alive," since it was missing several key components for life.
In a supplementary article for Nature Microbiology, University of Manchester biologist James McInerney suggested that LUCA may have been the only creature to survive an ancient population bottleneck. The fact that its genes are present today was a matter of sheer luck.
Regardless, the paper's findings are a net contribution to the deeply murky field of evolutionary biology. There isn't a way to fully prove that LUCA lived in hydrothermal vents or fed off iron deposits, simply because we currently don't have a way to recreate the origins of life. But the new findings shed much-needed light on an era of prehistory that has, for ages, remained uncertain.
"The goal of evolutionary biology is to understand the history of the organisms that we know," Bill Martin, a microbiologist and the study's co-author, told the Washington Post.
"When we're done with that we can worry about the ones we can imagine."