Life of a Lab Rat

The basic unit of life as we know it is the cell. Cells are essentially little bags containing DNA, the genetic material. The bag is made from fatty acids (oil) and some other greasy chemicals.

Oil and water do not mix. But fatty acids have two parts: a 'headgroup' that likes water and a greasy 'tail'. Because of this, if you took a drop of olive oil, say, and shook it up with the right amount of water you would get a suspension of little oily bags, or 'vesicles'. The greasy tails stick to each other and the headgroups stick out into the water. If everything is just right, the fatty acids can arrange themselves so that water is also trapped inside the bag. Think of a balloon, with the rubber being the fatty acids, with air (or water) inside and out.

If you added water-soluble chemicals (such as DNA) before shaking you could get some of them trapped inside the oily bags, and this is essentially how we think the first cells formed on the surface of the juvenile and excitable early Earth.

This is a nice little model, but then we run into problems. DNA consists of two strands that bind quite tightly to each other. They need to be separated before they can be decoded or copied (which needs to happen to make more DNA, to make more cells). Today (and for the last couple of billion years) we have specialized proteins that take care of the business of separating the two strands and building new DNA and RNA. But what happened before these proteins evolved?

One theory is that variations in temperature forced the two strands to separate, so that new DNA precursor molecules ('nucleotides') could assemble on them and react to form new DNA strands. When the temperature drops the new DNA double strands bind nice and tightly again.

However, fatty acid vesicles—our oily bags—are pretty sensitive things. They get upset if the pH is wrong, or if the wrong type and amount of other chemicals is present, or their concentration is low; if there are not enough fatty acid molecules in a certain volume the bags simply pop. Which is sub-optimal for the continued existence of our proto-cell. People assumed that the vesicles would also pop if the temperature changed too much, which puts the kibosh on the theory that thermal cycling could have been how DNA copying evolved. People (like me) who have tried to make fatty acid vesicles in the lab know all about these problems.

But two coves at the Howard Hughes Medical Institute in Boston Lincs Mass have actually done some rather interesting experiments (Open Access article). By making the fatty acid tails slightly longer the vesicles turn out to be surprisingly stable. They can be cooked at 100°C and still hold onto their cargo of DNA.

Simple prebiotic model membranes are clearly more robust than previously appreciated, allowing [for the] uptake of critical nutrients without the loss of larger entrapped material such as oligonucleotides

In other words, you can take these simple bags of DNA, heat them up to separate the strands and not lose anything. What's more, if there are nucleotides (our DNA 'building blocks') floating around outside they will slip into the vesicle at these high temperatures and get trapped inside—where they can stick to the separated strands and make new DNA.

Essentially, then, repeated heating and cooling of vesicles containing double-stranded DNA enables strand separation and re-annealing without destroying the vesicle. These vesicles also take up nucleotides at these higher temperatures, providing a potential mechanism for how proto-cells could obtain nutrients before they got around to evolving the transport proteins that exercise membrane biochemists so much today.

And where on the early Earth might we find such temperature variations? Apart from the diurnal day/night schtick we've got going here, hydrothermal vents and hot springs have long been considered to be candidate sites for early evolution. Cell warms up near the vent or spring, is carried away, cools down, drifts back by convection. Rinse, lather, evolve.

Of course, this doesn't mean we know how life did evolve, but it's a pretty convincing theory of how it could have.

S. S. Mansy, J. W. Szostak (2008). Thermostability of model protocell membranes Proceedings of the National Academy of Sciences, 105 (36), 13351-13355 DOI: 10.1073/pnas.0805086105