Came life from space?

Miklos Cserzo miklos at blue.incm.u-nancy.fr
Tue Oct 10 11:08:57 EST 1995

On Sun, 8 Oct 1995, Bill Pearson wrote:

> In article <EDDY.95Oct7085053 at wol.wustl.edu>,
> Sean Eddy <eddy at wol.wustl.edu> wrote:
> >
> >This is no problem, so long as the origin of life is probable, or the
> >Earth was lucky. But if it turns our that there just has to be a few
> >billion years for a massively complex life form like the last common
> >ancestor of Earth life to arise, it might become necessary to explore
> >the possibility that life arose on another, older world.
> >
> >Not that the hypothesis is testable, mind you.
> 	Of course it is!!!!  There are a number of proteins (glutamate
> dehydrogenase is perhaps the best candidate) with rates of change that
> allow us to look back much longer than 10By, the putative age of the
> universe.  See:
> %A R. F. Doolittle
> %A D. F. Feng
> %A M. S. Johnson
> %A M. A. McClure
> %D 1986
> %J Cold Spring Harb. Symp. Quant. Biol.
> %V 51
> %P 447-455
> %T Relationships of human protein sequences to those of other organisms

I wouldn't think that the interpretation of the sequence alignment data 
in terms of million years is a good idea. The mutation rate depends on 
factors like temperature and level of cosmic radiation. These - most 
probably - were different on the ancient Earth. Moreover organisms are 
protect their genome with a repairing apparatus against the mutations. 
This machinery - in turn - is also subjected to the evolution, i.e. its 
efficiency have been changed (improved) in the course of time. Thus one 
can be sure about the evolutionary distance of the sequences in terms of 
percentage identity (or PAM etc.) but not on the real-time scale. This 
later may be wrong by orders of magnitude.

> All we have to do is sample a life form from outer space, and the
> question could be answered quite quickly (I would be convinced if they
> had the same 20 amino acids I think).

Yes, that would be a fair test.

Miklos Cserzo.

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