We know that ancient organisms crawled out of the sea onto land, not the other way round. We know that the earliest life predates the existence of true continents. But life might just have began on land after all...sort of.
The secret is volcanic pumice. This type of rock is one of the very few that can float, and it might have served as a raft for emerging microorganisms in the primordial oceans some 3.5 billion years ago. Oxford professor Martin Brasier explains:
"Not only does pumice float as rafts but it has the highest surface-area-to-volume ratio of any type of rock, is exposed to a variety of conditions, and has the remarkable ability to adsorb metals, organics and phosphates as well as hosting organic catalysts, such as zeolites. Taken together these properties suggest that it could have made an ideal 'floating laboratory' for the development of the earliest micro-organisms."
Pumice is particularly appealing because it goes through so many unusual stages - first it's erupted by a volcano, then it floats along the water until the currents bring it to a shore, where it is beached in the tidal zone close to land. That would have given pumice rocks lots of opportunities to pick up the raw materials needed for life, and unlike other environments it would have provided any potential microorganisms with many different, potentially favorable conditions. Brasier continues:
"During its lifecycle pumice is potentially exposed to, among other things, lightning associated with volcanic eruptions, oily hydrocarbons and metals produced by hydrothermal vents, and ultraviolet light from the Sun as it floats on water. All these conditions have the potential to host, or even generate, the kind of chemical processes that we think created the first living cells."
There's no proof of it just yet, but the basic science is sound, and the researchers are hopeful that they can find evidence of such pumice rafts in the fossil record. There's also an experimental component to their search - they can simulate the cycles of heat and radiation that the ancient pumice would have experienced, and see whether new catalysts and compounds on the road to life emerge.