12 May 2010

The Adam and Eve Cell

While they are ways to exchange genes other than from parent to child, a new statistical analysis of genetic material shows that:

A universal common ancestor is at least 102,860 times more probable than having multiple ancestors. . . [The study] selected 23 proteins that are found across the taxonomic spectrum but have structures that differ from one species to another. He looked at those proteins in 12 species — four each from the bacterial, archaeal and eukaryotic domains of life. Then . . . performed computer simulations to evaluate how likely various evolutionary scenarios were to produce the observed array of proteins. A model that had a single common ancestor and allowed for some gene swapping among species was . . . better than a simple tree of life. Such a scenario is 103,489 times more probable than the best multi-ancestor model.

In other words, while the tree of life probably has intersecting branches, the base of the tree is probably a simply ancestor to descendant system, rather than one with multiple ancestors.

FWIW, I'm skeptical of the validity of the results, because it is plausible to me that the chemical paths that can give rise to life may be highly constrained by the laws of physics and strongly favor a single initial outcome under the conditions present when life first arose.

So, it is plausible to me that multiple precisely identical organisms could have formed at the same time. This would look statistically precisely like multiple identical descendants of a single common ancestor. Stong bio-chemical constraints could have prevented deviations from that single common ancestors from reproducing.

In other words, I'm concerned that the computer simulation allows for variation that is theoretically, but not physically, possible, while containing lots of paths that would be dead ends in real life.

This is especially a concern the closer you get to the base of the tree. The more complex and habitat specific an organism is, the less likely it is that the exact resulting organism was the only possible outcome of natural selection. But, I could easily imagine, for example, that there is no physically possible alternative to an RNA world scenario in anything remotely like the conditions in which life arose on Earth and that there are even far more limited number of RNA combinations that could work even in that scenario.

Alternately, it is also plausible that there are only two biochemically viable ways for life to evolve that are chirally opposites of each other, but are otherwise identical. In this scenario, two groups of identical organisms could have developed independently, one group of which would like like a common ancestors of all living things, and the other group of which would be crowded out and vanish swiftly after the prevailing group took hold. This early two possible ancestral type model would look like a single common ancestors after the fact and the winner in the early era might simply be a matter of random chance.

Why care? Because the distinction is very important to what kind of extra-terrestrial life might exist.

If all life has biochemical unity only because it has a single common ancestor, then extra-terrestrial life might be very different from life on Earth at a fundamental biochemical level (and an organism's genome is much more heavily devoted to its core biochemistry than to its surface morphology, which is just the icing on the cake in terms of the amount of genetic material involved).

On the other hand, if there is one strongly biochemically favored route to life in a particular environment, and only a minority of environments that exist in the Universe have any biochemically possible path to life, then life everywhere in the Universe will fall into a finite number of different possible types at a fundamental level. And, in places that have had historically similar environments, there may be only a single type of fundamental biochemistry for all possible living things. If that is the case, then if traces of life were found on Mars, they would look genetically, at some taxonomic level, as if they were descended from life on Earth, and due to selective pressures would look much more closely related than random chance would suggest.

If a chiral version of the single perferred route theory was correct, traces of life on Mars might be biochemically identical to life on Earth, or might be its chiral opposite at a fundamental biochemical level.

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