Meet the surprisingly complex ancestor of all life on Earth

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It may be hard to imagine complicated life forms on early Earth. Oxygen was low and asteroids may have been pelting the baby planet’s surface. Despite those harsh conditions, a microbe may have persisted, giving rise to all life we see today.

This is the Last Universal Common Ancestor, or LUCA.

In the most extensive analysis of the organism to date, scientists propose in a new study that this hypothesized ancestor was more sophisticated than previously known – thought to possess an immune system to fight off viruses, for instance.

The team said LUCA appeared around 4.2 billion years ago, shortly after Earth was thought to be habitable, suggesting it evolved even quicker than previous estimates and survived through tumultuous times on the planet. The findings could help us understand more about life on this planet and others: What if it doesn’t take long to kick-start life?

“All life on Earth is related, from all animals, plants, fungi, protists to every single type of bacteria that exists. They all have this common ancestor,” said Edmund Moody, lead author of the new study. “We have this idea that this was a fairly complex organism, already possibly by the time of like 4.2 billion years ago.”

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A fast evolution

Scientists as far back as biologist Charles Darwin have proposed that all living things evolved from a common origin, which sits at the base of the family tree of life. The organism would possess genes and components present in all living forms today, such as ribosomes, certain proteins or enzymes important for energy, Moody said.

But LUCA may have been more complex than they previously thought, the authors found. They inferred it had an immune system that fought viruses and found evidence suggesting it contained genes to protect against ultraviolet damage and lived at the ocean surface.

Our late ancestor might not have been alone either, Moody said. Although not a direct conclusion from the study, he suggested LUCA probably would have been part of an early ecosystem of many organisms, competing to stay alive. But the other organisms probably died out, which would explain why they wouldn’t be represented in any modern genomes or fossils.

“LUCA was a very complex cell, with a genome similar to modern bacteria (which we think of as simple, but from a molecular biology perspective are very complex),” Aaron Goldman, a biologist at Oberlin College who was not involved in the study, said in an email. He called the study a “breakthrough in the discipline” in a review article of the study.

Previous research dated LUCA as young as 3.5 or 3.8 billion years ago, so the new timeline accelerates evolution by a few hundred million years.

Although Earth was formed nearly 4.6 billion years ago, scientists think our planet wasn’t cool enough for habitable environments until about 4.3 to 4.4 billion years ago, said Goldman. If LUCA was indeed around 4.2 billion years ago, as the study suggests, this dating would rewrite our understanding of how fast life can emerge under the right conditions.

“That is a lot of evolution to happen within 100 million years or less,” Goldman said.

The new timeline and details can be chalked up to more advanced analysis methods available today. In the new study, the team of 19 scientists used a combination of genetic analysis and fossil records to determine the age of LUCA and its characteristics. They first compared genes in modern genomes of bacteria and archaea to determine which gene families were present in LUCA. They estimated LUCA’s genome size, the number of proteins it encoded and its metabolism.

“The computational model is kind of working its way backwards to say that these things are slightly different and, based on our model of molecular evolution, they probably share a common ancestor,” Moody said. “You work all the way back and eventually you’re going to get a common ancestor of everything, which is LUCA.”

The team separately performed an analysis using a much smaller number of genes that they thought duplicated before LUCA, which they calibrated with fossils to get its age.

Moody said the research is probably the “most ambitious” attempt to characterize and date LUCA.

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Could LUCA have survived during the late heavy bombardment?

Not everyone is convinced of this new timeline and depiction of LUCA.

Patrick Forterre, a researcher at the Pasteur Institute who was not involved in the study, said “the age indicated by the authors means that LUCA lived during the early heavy bombardment,” a hypothesized violent period in Earth’s history around 4.1 to 3.8 billion years ago – when asteroids and comets battered the planet. Forterre said it seemed very unlikely to him that LUCA would survive such harsh conditions.

Some scientists have said that the heavy bombardment may never have occurred. Or perhaps the heavy bombardment stage wasn’t “necessarily a planet-sterilizing event,” said Moody. “I think that’s an active area of research.”

In fact, new research released Monday suggested that organisms around that time could have received the necessary ingredients for life through lightning hitting the ground. Mimicking cloud-to-ground lightning in a lab experiment, scientists found that carbon and nitrogen in the air were converted into biologically useful molecules in high enough concentrations to spark or sustain life on Earth at that time.

Haihui Joy Jiang, lead author of Monday’s study and a researcher at Harvard University, didn’t comment if her study supported this new LUCA theory but said this energy source could have been in play more than 4 billion years ago. That date matches up with the revised age of LUCA.

“It’s perfectly compatible with our reconstruction of LUCA because LUCA would have relied upon things like lightning to produce nitrogen,” Moody said of the new research. “If LUCA was around at that time, it would have absolutely been gobbling that up and using that to make its DNA and its proteins.”

If such advanced organisms could find ways to survive early in Earth’s history, we could be more confident in our hunt for life on other planets.

“The chances of an origin of life on any planet might be higher than we previously thought as long as the right conditions are met,” said Goldman.