NASA announced several days ago, an upcoming press conference
that would talk about "alien life". This is big news. They have sent more than one mission to Mars looking for life, water and extraterrestrials. But several people, including the late Sir Fred Hoyle
, have suggested that alien life was coming here, which would save a lot on expenses. The question became, how do we know it is alien?
, working from a suggestion from my colleague Richard Hoover
, has been looking for life
on our planet that does something different from all other Earth life. The argument is a bit indirect, but here's the gist of it. If life exists on our planet by accident (standard Darwinian hypothesis) then it must be somewhat probable. That is, if it is as improbable as Hoyle calculated, 1 chance in 10^40,000, then it is so improbable as to never happen. But it must have happened (by accident, we've assumed) and so it must be probable, say 1 in 10^20. (This step involves two miracles, making it less robust than a one-miracle theory.) Well if it is probable, then life must be popping up all over the galaxy, wherever there's the right conditions for it. However, many of the parameters of life appear arbitrary, like the code that connects DNA codons to protein amino acids. So it would seem that if the right conditions included lots of arsenic and no phosphorus, then naturally it would be arsenic-DNA. In addition, it should probably show other signs of extraterrestrial source, perhaps a different DNA code, and different ATP energy source, etc.
Then if this alien life were to land on Earth, say from a comet as Hoyle suggested, it would grow independently of Earth-life, being inedible to Earth creatures, and itself unable to eat them. In fact, arsenic is a poison for most Earth life, since it interferes with the phosphorus. Well this big population of alien bugs would then form a "shadow biosphere" that may go undetected, since it wouldn't respond to any of the usual nutrients we assume are necessary.
With that sort of a justification, Paul Davies started talking about "life as we do not know it
", and Felisa Wolfe-Simon
started looking for arsenic bugs in Mono Lake
, California. The idea was that alien bugs arrived at Earth perhaps on a comet which burned up in the atmosphere releasing a fine dust that settled on the surface. When bugs in the dust found a good spot to grow, they started multiplying without disturbing the Earth life. Mono Lake, like the Dead Sea, has no outlet, so it is chock-full of minerals washed down from the mountains, among which are loads of arsenic. Felisa would take a sample of mud, add some sugar and nutrients, but no phosphorus, only arsenic to the growth medium. If the solution looked cloudy, then bugs were growing in it. She would then take a small amount of this solution, and put it into a fresh growth medium with no phosphorus but lots of arsenic. By diluting her growing solution many times, she could be certain that there was no phosphorus in the final cloudy material, and she had successfully cultured an arsenic bug,
and NASA trumpeted
the press release.
So far this sounds a lot like the paper
Paul Davies published two years ago, in which he successfully grew bugs in a growth medium of mirror-image amino
acids and sugars. All life on this planet uses left-handed or L-amino acids and right-handed or D-sugars. Davies mixed up a growth medium of D-amino and L-sugar, and found several bugs that grew on it. They were a previously unknown species of bug, which isn't too unusual, but the rest of them was boringly traditional. They evidently had an extra enzyme that could munch D-amino acids, and thus outgrew the competition with this bit of help. Davies got a publication of it, but it didn't convince anyone that he had found aliens.
Felisa's bug is different, though, because DNA uses phosphorus as the critical bond between "rungs" of the double helix. If you think of a ladder as made of two rails and many rungs, then phosphorus is the atom that permits three connections--an up-rail, a down-rail, and a rung. This is the backbone of the DNA molecule, and when you combine two strands of DNA in a double helix, you have two rails like a railroad, twisting through space. Many of the molecular motors that copy DNA, repair DNA, make RNA, modify transcription of RNA etc, run up and down on these rails. And because this is such a long molecule, they have to move fast.
But the size of the phosphorus +5 ion is about 34 picometers,
whereas the size of the replacement arsenic +5 ion is 47 picometers
. This is a change in size of 38%. Replacing the phosphorus in these rails (black dots in graphic) with arsenic is like changing the gauge of the railroad. And if you hit that change of gauge at 30 mph, there's going to be a train wreck. None of the critical DNA machinery is going to like swapping out even one phosphorus atom, much less all of them!
Superficially it would appear that this bug is using alien DNA, which is completely incompatible with phosphorus based DNA. Davies appears to have found his alien life. Or has he?
It seems (without having read the Science paper yet) this bug prefers to grow in phosphorus growth medium, and only grows in arsenic when it has to. And it would be my educated guess, that it has the same DNA code, the same ATP shape as normal Earth life, only switching to arsenic when it has to. In other words, I think we are seeing highly adapted Earth life, and not alien extraterrestrial life.
To prove this will require lots of more work in transcribing the DNA of the bug. This can be done in a few weeks with modern genome equipment, but only if the usual phosphorus enzymes work on the arsenic version. If they don't then it could be years before we have the genome transcribed. Looks like it might be a long while to resolve whether we are seeing an alien or an adaptable Earthling.
Why am I so sure it is adaptable Earthling? Because I believe Hoyle was right, that the probability of life is worse than 1:10^40000. So life only arose once, and not by accident. That makes the probability of finding a different DNA code or a "shadow biosphere" impossible. Instead, we find highly adaptable life designed to function in different environments, but sharing most of the common engineering solutions. I predict that when the genome is transcribed, we will find extra genes for phosphorus railcars, for arsenic railcars (with slightly narrower gauge wheels) and the additional machinery needed to swap between them. Then when the environment gets too much arsenic, the extra DNA starts making arsenic rails, arsenic railcars, arsenic engines, and when it has enough, it assembles them on one side of the cell, splits in two, and starts up the arsenic only version. Likewise when phosphorus becomes available, the bug will likely do the reverse, starting up the phosphorus version.
My skeptical NASA colleague suggests that instead the cellular solution might be to find railcars that can run on both kinds of track, so that rather than have two optimized solutions, there may be a less-optimized version of railcars and engines that can run on both tracks.
Either way, it will be an adaptation and not an alien we are observing. For not only is this not "alien", and not only is life improbable, but Darwin (and the Neo-Darwinian Theory that replaced it) could not allow life to modify its own genetics. That would be Lamarckian
, and would introduce purpose into evolution. The slightest tinge of purpose would poison random mutation and reintroduce "real" design, rather than "apparent" design. Now we have a bug that not only can mess with its own DNA, it can swap out atoms from its DNA backbone, changing horses in the middle of the stream.
The Darwinian prediction has failed yet again.
I've read the paper and didn't find any surprises, except that their extended dilutions still only resulted in bugs that were 85% As and 15% P. So they never did remove all the P. However they used 3 separate techniques to demonstrate that As really was in the backbone of the DNA. So it appears that we do have a rail gauge problem.
Some have asked whether life could have started with As and later acquired P abilities. A table of the elemental abundances in the solar system (which are very close to the elemental abundances in the galaxy) demonstrate two things:
1. Of the six or so elements necessary for life: H, C, N, O, S, P, phosphorus is the hardest to get. That' s why dishwasher detergents have suddenly stopped using phosphate (and consequently leave scum on the dishes) because the phosphorus was ending up in the lake/sea and causing algae blooms.)
2. Arsenic is even harder to find than phosphorus, coming in around 1/10000 the availability. There are probably less than a dozen places on Earth where As outnumbers P. Chances are that life started with P than with As.