A Glimpse of LUCA, Life’s Last Universal Common Ancestor

There is overwhelming evidence to support the idea that all modern life evolved from a common ancestor in our distant past (called the last universal common ancestor or LUCA). Now a new study out in PLOS Biology by Sojo and coworkers suggests how this primitive form of life might have survived without some of our sophisticated cellular machinery.

Now this study is not about how life arose from non-life. That is a mystery that scientists have yet to solve.

What this study describes is a form of life that existed in that transition point between the very start of life and the modern world. This version of LUCA managed to survive in a very specific location, probably deep in the sea, but was trapped there until it evolved a few modern traits. If the study pans out, we will have gained a new fundamental understanding about how early life survived on Earth and how it spread.

Using Modern Life to Reconstruct LUCA

LUCA was way too fragile to provide any sort of fossil record for us to study (or at least none have been found so far). But that does not mean we have no chance of figuring out a few things about it.

We can make some educated guesses about what early life must have looked like by comparing various modern forms of life. For example, if all life shares something like a ribosome, then LUCA must have had a ribosome too. But if something like copying DNA is very different in different organisms, then those exact processes may have developed after life started to evolve in separate directions.

The two most ancient kingdoms of life are bacteria and Archae. (Eukaryotes like us came later most likely in a merging of these two ancient life forms.) In this study the researchers focused on how the membranes are different between these two ancient lineages and used these differences to infer what LUCA’s membranes may have been like. And from that they were able to come up with how LUCA may have thrived in alkaline hydrothermal vents at the bottom of the sea without some of the complicated machinery modern cells need to survive.

Leaky Membranes the Key

One of the main components of life is some sort of membrane to contain all of its chemical reactions. Given how important membranes are you might think that all modern life shares very similar membranes. But they don’t.

Archae and bacteria have fundamentally different membranes which suggests that modern membranes evolved after these two split away from LUCA. With a little biochemical detective work, the authors worked out what these early membranes probably looked like. And unlike modern beasts, LUCA almost certainly had a leaky membrane.

Now this doesn’t mean its membrane was like a sieve with anything and everything leaking out.  Instead it means that small ions could easily pass through such a membrane.

This would be disastrous today. Cells are set up so that they create an environment where there is a big ion difference on each side of key membranes. This difference is how cells make the ATP that pretty much provides all the energy for the cell.

A leaky membrane would make it very difficult to set up such an ion gradient. As the cell pumped in ions they would quickly be lost as they leaked out of the cell. This would inevitably lead to an equilibration of ions and as the authors so eloquently note in their article, “Equilibrium is death.”

If Sojo and coworkers are right, LUCA was able to use its leaky membranes to its advantage.  In alkaline hydrothermal vents (which are different than the black smokers most people know about) there is a steep pH gradient over very short distances. What this means is that there could easily be a natural ion gradient on either side of a cell!  It also means that LUCA didn’t need to have evolved any of the ion pumps we have now.  This form of life could survive with a much simpler set of cellular machinery.

The authors hypothesize that LUCA sat at a certain position with lots of H+ ions on the acidic side and lots of OH- ions on the other. The H+ leaked in one side and were neutralized on the other by any OH- ions that came in.

The researchers model this sort of system and show that a gradient of just 3 pH units on either side would be enough for LUCA to make enough ATP to do the things a cell needs to do. And this sort of pH gradient is easy to get in the micropores of alkaline hydrothermal vents.

Of course, now the poor thing is stuck in that sweet spot at the bottom of the ocean. To spread it would need to evolve less leaky membranes and a way to pump in ions. I don’t have time to go into it here but the researchers have a plausible explanation for how this may have happened as well.

This is when bacteria and Archae would have started going their separate ways.  Each would have tweaked its membranes in different ways to make them less leaky allowing each to set sail and colonize the Earth.  And of course, eventually combine and evolve to become us!

Here is a true ancestry search. By looking at distant cousins, these scientists were able to make reasonable guesses about what our common ancestor might have looked like. A far cry from modern life but not as far off as we might have thought.