The fossil record reveals the last 3.4 billion years of life on Earth. But before then, when life first emerged, we have next to nothing. The consensus view is that life began in the oceans... but there might be another, weirder, possibility.
A team led by University of Osnabrück biophysicist Armen Y. Mulkidjanian has just published a paper in Proceedings of the National Academy of Science, or PNAS. In the paper, Mulkidjanian and his team argue that the nature and composition of cells might be evidence that their most ancient ancestors emerged from somewhere very different from the current preferred candidate, which are thermal vents at the bottom of the ocean.
The idea is that the first cells must have been capable of surviving in their surrounding environment — otherwise, we wouldn't be here. Because those first cells were primitive and lacked the more sophisticated survival mechanisms that later cells evolved, that limits the places these cells could have emerged to environments that were a close chemical match for the cells themselves. Assuming the composition of the modern cell is a good approximation of its ancient counterpart, that can tells us where cells must have first emerged.
Mulkidjanian and his team describe the cell's composition in the introduction to their paper:
All cells contain much more potassium, phosphate, and transition metals than modern (or reconstructed primeval) oceans, lakes, or rivers. Cells maintain ion gradients by using sophisticated, energy-dependent membrane enzymes (membrane pumps) that are embedded in elaborate ion-tight membranes. The first cells could possess neither ion-tight membranes nor membrane pumps, so the concentrations of small inorganic molecules and ions within protocells and in their environment would equilibrate. Hence, the ion composition of modern cells might reflect the inorganic ion composition of the habitats of protocells.
As they argue at the outset, the molecules and compounds found inside the cell do not appear to fit with saltwater environments. Instead, they say it's much more likely that the first cells emerged in an environment much like that of modern hot springs:
Geochemical reconstruction shows that the ionic composition conducive to the origin of cells could not have existed in marine settings but is compatible with emissions of vapor-dominated zones of inland geothermal systems. Under the anoxic, CO2-dominated primordial atmosphere, the chemistry of basins at geothermal fields would resemble the internal milieu of modern cells. The precellular stages of evolution might have transpired in shallow ponds of condensed and cooled geothermal vapor that were lined with porous silicate minerals mixed with metal sulfides and enriched in K+, Zn2+, and phosphorous compounds.
This idea resembles Charles Darwin's initial conception of the origin of life, in which he saw the first cells emerging from "warm little ponds" and ultimately making their way to the oceans. It's a comparison the researchers explicitly make in their paper, and they go one step further by naming the specific "pond" - the thermal springs of Kamchatka in far eastern Siberia have just the right chemical mix to account for the potassium-rich composition of cells.
It's an intriguing idea, and we've considered similar land-based hypotheses before, but it's going pretty strongly against the current scientific consensus. As New Scientist reports, some of the scientists they contacted about the paper "labelled Mulkidjanian's ideas absurd and declined to comment."
Indeed, there are two major issues with the hypothesis as it currently stands. For one thing, any land-based life around 3.8 billion years ago would have been pummeled by meteors during what's known as the Late Heavy Bombardment, an event that the ocean would have protected any vent-based cells from.
Perhaps even more crucially, the paper assumes that the earliest cells could have done nothing to survive in salty environments, but extremely primitive microbes known as methanogens and acetogens have shown they can pump out sodium ions to survive in such waters. It may well be less of a jump to argue that the very earliest cells could also pump out these ions than it is to move the origin of life from sea to land.
The truth might ultimately end up somewhere in the middle, and we might actually end up right where Darwin suspected all along - the warm little ponds. These freshwater environments would have had salt concentrations low enough to let cells evolve more comfortably while also keeping them away from the harsh exposure of a land-based existence. Unfortunately, we have so little evidence to work with from these most ancient times that we may not be able to move much beyond speculation for the foreseeable future.
For more, you can check out Mulkidjanian and his team's fairly readable original paper here.
Via New Scientist. Image of Ketetahi thermal springs in New Zealand by Neville10 on Flickr.