Evolution of life [was Re: Why Would Aliens Hide?]

Robert J. Bradbury (bradbury@www.aeiveos.com)
Thu, 25 Nov 1999 11:39:29 -0800 (PST)

... Eric Watt Forste <arkuat@idiom.com> wrote:

> Right, but biota aren't based on heavy metals. Most carbon and oxygen
> are produced by *much* slower processes, principally planetary
> nebula ejection and white-dwarf novas. These events happen
> gigayears or tens of gigayears after star formation.

Its an ongoing process, and the lighter elements are produced in much greater abundances than the heavy elements.

Astronomy Today, Table 21-1, Cosmic Abundances (current era):

  Elemental Group                % abundance by number
  H                                     90
  He                                     9
  Li Group (7-11 (p+n))                  0.000001
  C group (12-20 (p+n))                  0.2
  Si group (23-48 (p+n))                 0.01
  Fe group (50-62 (p+n))                 0.01
  Middle wt. group (63-100 (p+n))        0.00000001
  Heavy wt. group (>100 (p+n))           0.000000001

Heavy stars (> 8 M_sun), have a layered structure (from the outside) of: H -> He -> C -> O -> Ne -> Mg -> Si -> Fe Presumably much of the Fe gets dumped into the black holes or the neutron star for lighter stars, but not all of it since some of it gets released into the interstellar media. Since these large stars only have lives of millions to tens of millions of years the interstellar dust gets enriched with the materials of life *very* rapidly after the formation of the galaxy.

While the distinguishing feature between population I and II stars tends to be their location and velocity (II are found more in the halo and have higher velocities, probably due to their being older and having had more change sling-shot encounters), they also differ in metal content. Pop. II stars are very deficient in elements heavier than He, while Pop. I have approximately solar abundances of metals.

So I would propose that almost all population I stars should be potentially capable of forming planetary systems on which life could evolve.

[Now, of course it goes without saying that the astronomers would start pulling out their hair if one were to suggest the population II stars aren't old, but are instead leftovers from SI mining activities.]

On Wed, 24 Nov 1999 CurtAdams@aol.com wrote:
> More relevant might be concentrations of elements whose concentrations often
> limit on earth; nitrogen and phosphorus spring to mind. (Iron can be
> limiting in the ocean but that's more of a solubility issue.) Life uses a
> lot of trace elements: I wouldn't think concentrations of things like
> selenium would be limiting but I'm not really sure.
> I think nitrogen is the most limiting element since life uses a lot of it,
> it's relatively rare, and since it's mostly gas it's very prone to get lost.

N2 is an interesting element to look at for rarity, particulary from the perspective of why you don't find much on Mars or Venus (relative to say CO2). On Mars, I can understand losing N2 before CO2, but on Venus there ought to be a fair amount.

An article (Terraforming with Nanotechnology, JBIS, 47:311-318, 1994), discusses this: "The availability of nitrogen seems to be the most serious constraint on the eventual establishment of a completely Earth-like environment on Mars." [That and the complete dismantlement of the planet....]. They also observe, "If underground stores of nitrogen do exist on Mars, just what form they take is a mystery. Nitrate minerals are one possible store. Although nitrate minerals do exist on earth, they are rare. Due to their solubility they are only found in the most arid regions of Earth such as the deserts of Chile. Even though Mars is now in a desert state, its history as a water planet makes the existence of large stores of nitrates doubtful." [I'm not sure why this would be true, if N2 gets locked up in soluble nitrates, it would seem that it would be deposited as the H2O evaporates or still be present in any frozen ice.]

This gets particularly interesting in light of the abundance of elements in the solar system (from Physics & Chemistry of the Solar System, J.S. Lewis, after Grevesse & Anders (1988)) and Nanomedicine, Chapter 3, Table 3.1:

  Element      Solar Abundance  Atoms in Body
  H            2.8 x 10^10       4.22 x 10^27
  O            2.3 x 10^7        1.61 x 10^27
  C            1.0 x 10^7        8.03 x 10^26
  N            3.1 x 10^6        3.9  x 10^25
  Fe           9.0 x 10^5        4.5  x 10^22
  S            5.2 x 10^5        2.6  x 10^24
  Ca           6.1 x 10^4        1.6  x 10^25
  Na           5.7 x 10^4        2.5  x 10^24
  P            1.0 x 10^4        9.6  x 10^24
  K            3.8 x 10^3        2.2  x 10^24
  Zn           1.3 x 10^3        2.1  x 10^22
  Cu           5.2 x 10^2        7    x 10^20
  Se           6.2 x 10^1        3    x 10^18

At any rate, after going through the data entry and calculations... Relative body enrichment: P > K > Rb > Cl > Ca > Cd > Br ... Relative body depletion: Cr < Ni < V < Co < Mn < Al < Se < Fe ... Relatively equivalent: C ~= O ~= I ~= Na ~= Zn ~= N (decreasing order but within a factor of 6.3 of each other)

I'll have to dig a little further to do the calculation relative to the Earth's crustal or salt water abundances (which may make more sense).

Of course, the ratios will probably vary from species to species, particularly in widely separated branches of the animal & plant kingdoms.

Now, the interesting thing about most of the heavier elements is that they are being used in small quantities for specific purposes. S gets used for disulfide bridges, P is used as an energy carrier, Fe as an O2 carrier or catalyst, Zn as a protein "cross-linker" (Zn-finger transcription factors) and catalyst, Cu & Se as catalysts, etc. I've never heard anyone suggest that the heavier elements are *required* for life. I've given some thought to whether we could re-engineer life to remove the metals (you especially want to remove Cu & Fe if you want to minimize the free radical damage to DNA in the long term). So far, I haven't come across any strong reasons why the functions done by heavy "metals" in our body could not replaced by protein only constructs. I will admit that you may suffer some reductions in reaction rates, but it isn't clear that there are reactions that simply "cannot" occur without the presence of metals.

So even though we require heavy metals (from supernovas) it isn't clear that all forms of life must. I suspect that life gets started without using metals (other than HOCN) and then incorporates them for specific functions. To use "heavy" metals effectively, you have to evolve transport and storage management systems. Given some really big computers for molecular dynamics simulations, I'm pretty sure we could rapidly develop proteins/enzymes that did not require metals.

So, in solar systems where heavy metals are not as abundant (very early population I stars) life might certainly evolve. It might take longer, meaning it might take 5-6 billion years instead of 4 billion, implying that it would want to occur around a smaller, longer lived star, but those stars are more abundant so overall, so things probably average out. A more critical problem, IMO, is whether early population I stars would have enough heavy material to form solid planets as are found in our solar system. If not, then you have to discuss evolution of life on moons around gas giants or on comets which rapidly gets highly speculative.