Sharov and Gordon reach this conclusion about the origin time of earthly life by doing a backwards extrapolation. They suggest that the growth of life's complexity follows a law of exponential growth similar to Moore's Law, the law that the number of components in a state-of-the-art computer doubles every 18 months. The scientists created a graph that plots on a logarithmic scale the growth of genome complexity in earthly organisms. They find that on this graph the growth of complexity pretty much follows a straight line. When one traces that straight line back to its beginning, you come to a point about 9.7 billion years ago.
A graph from the scientific paper (click to expand)
This conclusion is startling. The standard story is that life first appeared on our planet a few billion years ago, because of some lucky accident. When they started to put together this graph, the scientists must have expected the trend line to begin at a point about 4 billion years ago. Instead, following the trend line back to its beginning, they end up with a starting point 9.7 billion years ago.
Could it be, however, that the rate of evolution has not been constant? Some evolutionary scientists support a theory of “punctuated equilibrium,” in which species exhibit little change over very long periods, and then suddenly branch off into several new species. But Sharov and Gordon anticipate this objection. They argue that while one species may split into several in a fairly short time (geologically speaking), the overall growth in functional complexity does not have sudden spurts:
The reason why living organisms cannot increase their functional complexity instantly may be that it takes a long time to develop each new function via trial and error. Thus, simultaneous and fast emergence of numerous new functions is very unlikely. In particular, the origin of life was then not a single lucky event but a gradual increase of functional complexity in evolving primordial systems. Similarly, the emergence from prokaryotes to eukaryotes was not the result of one successful symbiosis, but may have involved as many as 100 discrete innovative steps.
But how do these scientists reconcile this starting point of 9.7 billion years for earthly life with the fact that scientists say Earth is only 4.6 billion years old? Do they suggest that Earth is twice as old as we thought? No. Instead Sharov and Gordon suggest that earthly life is descended from life that originated elsewhere.
But how could earthly life be descended from life that arose elsewhere in the universe? Not wishing to use the deus ex machina of extraterrestrial spaceships, Sharov and Gordon suggest a simpler idea: spores. The scientists put it this way:
Bacterial spores have unusually high survival rates even in very harsh conditions, and therefore, they are most likely candidates for interstellar transfer. Contaminated material can be ejected into space from a planet via collision with comets or asteroids. Then bacterial spores may remain alive in a deep frozen state for a long time that may be sufficient for interstellar transfer. Bacterial spores were reported revived after 25-35 million years of dormancy.
The basic idea can be summarized like this:
Phase 1: Billions of years of extraterrestrial evolution in some other solar system
Phase 2: Spore migration from other solar system to our solar system
Phase 3: Billions of years of earthly evolution
The idea of life naturally spreading across the stars may seem farfetched, but it seems more reasonable when we consider that according to astronomers many of the elements on our planet originated in other solar systems. Scientists say that all of the elements heavier than iron originated in supernovae explosions that occurred before our solar system formed. Those explosions of stars are believed to have created the elements heavier than iron, and spread those elements around into interstellar space. Scientists think that such explosions enriched with heavy elements the cloud of gas and dust from which the solar system formed.
With its emphasis on the long time needed for life to evolve, Sharov and Gordon's theory may help to supply an answer to Fermi's Paradox, the issue of why extraterrestrials have not appeared here or taken over our planet. If it takes something like 10 billion years for intelligent life to evolve from the most primitive state of life, that can help to explain why “they” are not here already. It could be that we are one of the first intelligent species to evolve.
The theory of Sharov and Gordon may have merit, but I don't think they have done enough to promote their “life before Earth” theory. Perhaps they need a more catchy marketing approach. To help them sell their theory, I have created the promotional poster shown below, which I give them permission to use. In this visual I combine elements of their theory with motifs from two of the most beloved science fiction series.