When, where, and how life first appeared are among science's biggest questions. A new model attributes life's formation to bombardment by meteorites. It also suggests there was only a short period during which life could have begun.
Natural selection provides a powerful force for life to grow and develop, but getting started required the bringing together of nucleobases to form what became RNA. The most popular suggestions for the location of this event are hydrothermal vents at the bottom of the ocean and what Darwin called a warm little pond (WLP). A nuclear reactor has recently been proposed as well, but it is yet to be properly reviewed.
According to McMaster University graduate student Ben Pearce, WLPs have several advantages: “Their wet and dry cycles have been shown to promote the polymerization of nucleotides into chains possibly greater than 300 links,” Pearce writes in Proceedings of the National Academy of Sciences. “Furthermore, clay minerals in the walls and bases of WLPs promote the linking of chains up to 55 nucleotides long.” Attempts to replicate the conditions around hydrothermal vents have produced RNA chains too short to be likely starter kits for life.
However, WLPs still need nucleobases to join together in the first place, and the atmosphere back then is not thought to have been something well-suited to their formation. Moreover, three sorts of nucleobases have been found in meteorites.
How likely then was it that nucleobases could reach Earth aboard a meteorite, survive passage through the atmosphere, and splashdown in a suitable pond where wet-dry cycles could cause them to join together to become life's first RNA?
We know the early Earth was peppered by meteorites, although the record of the exact rate has been lost to erosion and geologic forces. Pearce and his co-authors use the rate of cratering on the Moon to calculate the number of impacts the Earth experienced in its first few hundred million years and factored in that only carbonaceous meteorites are likely to be suitable carriers. In addition, only a few of those traveled slowly enough to not burn up in the atmosphere.
These estimates were compared with likely numbers of lakes and ponds on the Earth's very limited continental crust at the time.
The authors conclude that prior to 4.17 billion years ago, there were sufficient cases of suitable meteorites touching down in WLPs to make seeding credible. RNA formation needed to occur before the nucleobases seeped out of the pond, which required temperatures of 50-80ºC (122-176ºF), but that is considered likely at the time. The paper concludes that life could have begun within just a few cycles of rainfall and drought after a meteorite strike, particularly if sedimentation protected the nucleobases from UV radiation.