Just several years ago, when this blog already existed, I got converted to the idea of panspermia which says that the earliest forms of life were born somewhere in outer space before the Earth was formed – and they just found Earth to be a particularly hospitable destination where they could further evolve and flourish.
Now, the Physics arXiv Blog discusses the preprint
The argument is based on a computer science meme, Moore's law, that claims that the number of transistors on a circuit is exponentially increasing with the doubling time that is either 18 or 24 months or so.
Just for the sake of completeness, this is how the number of transistors grew from the 1970s:
Growing by a factor of 1 million (20 doublings) in 40 years translates almost exactly to a 2-year doubling time.
I think you will agree that the line – with a logarithmic y-axis – is indeed remarkably straight. The case of biology is designed to be as similar as possible. Instead of transistors, they count the functional genome's base pairs (bp). The result looks like this:
The functional non-redundant genome size apparently grows 10-fold in a billion of years or so.
In particular, the red dots are ordered truly linearly – the growth is exponential – and there's no doubt that if you extrapolate the number back to "the origin of Earth" 4.7 billion years ago, you still get a genome size above 10,000 base pairs.
The most obvious criticism you may raise is that this naive straight-line "Moore's law" just fails. The line is allowed to bend. Equivalently, the explosion of the genetic information in an animal could have grown exponentially but with a much shorter doubling time when life was really getting started. Maybe. However, note that you could raise the same objection in the case of the integrated circuits but Moore's law seemed OK even when computers were very, very young.
Even when Moore's laws in this particular form – one could of course guess that other precise quantities grow exponentially but this "transistor/base pair count" seems to be particularly well-behaved – are correct, a question is whether we may "prove" or at least find an argument that "explains" why the doubling time should be constant. I don't have such a "theoretical proof" and I am interested in it if you know one.
However, as I have previously stated, there is a reason why I think that the idea that "all steps to life had to occur on Earth" is probably wrong. It's just unnecessary and based on a one-size-fits-all egalitarian reasoning that has obvious limitations.
If you believe that even short DNA molecules were first born on Earth, you probably defend such a viewpoint by pointing out that our blue, not green planet is so unusually hospitable and obeys the conditions we need for life. So it had to be here.
This claim seems to treat the word "life" in a very sloppy, ambiguous way. It's the higher life similar to ours that needs the oceans to swim in and other achievements that Earth seems to boast and employ to defeat many competing tourist destinations. However, when we talk about the origin of life, we're not talking about the origin of mammals. The latter could have taken place on Earth but that does not imply that the former event had to occur terrestrially, too.
In fact, I find it pretty obvious that the concentration of the habitats to friendly hot spots is becoming more focused as one goes towards the higher life forms. If a professional (=paid) string theorist is picked as the prototype of the highest life form on Earth, its or his or her habitat is confined to several floors at Princeton, New Jersey, Cambridge, Massachusetts, and a dozen of comparably small places that are getting less and less characteristic. These are much more special places than the places where mosquitos and fungi flourish – which is almost everywhere on Earth.
If you revert this "increasing concentration of genetic capital" to reconstruct the distant past, it seems reasonable to think that the "optimum place" for some extremely old, primordial life forms was less localized than the Earth's surface. After all, it's probably not true that these life forms needed a nice amount of planetary gravity – gravity becomes almost irrelevant for very small cells etc. much like it is irrelevant in particle accelerators. Similarly for water etc.
I think that the volume of "cavities in rocks" and similar things that are flying around all stars in the Milky Way is probably higher than the volume right above the Earth's surface where we expected terrestrial life. In particular, even if you insist that the temperature is right, you don't need an Earth-like planet at the right radius that orbits a given star. The rocks may be just enough.
I don't want to be excessively specific about the "less demanding conditions" that the early life could have depended upon because I haven't spent too much time with thinking about all such possible conditions. But I surely do believe the general principle that the primitive life is less choosy and less picky than advanced life. This very general principle is enough for me to prefer the hypothesis that the very early life was born at rather generic extraterrestrial places and it just found the Earth to be a particularly hospitable place for a further evolution.
What do you think?