On 11/12/2024 07:32, Martin Harran wrote:
On Mon, 9 Dec 2024 13:57:43 -0800, erik simpson
<eastside.erik@gmail.com> wrote:
[snip for focus]
Self-catalyzing time for a strand of RNA is probably on the order of
minutes. A black smoker need only be present for few years, and the
early earth had a much hotter interior means that there were at least
millions of them. As SJ Gould remarked "life may be as common as
quartz". Indeed. All you need is hot water and a thermal or chemical
gradient and you're good to go.
If that is the case, why have we not seen any new life forms develop
from scratch in the last several billion years with every form of life
we know descending from a single origin?
I know the typical response is that in the early earth, there were
possibly numerous life forms with one dominant one devouring the
others but that seems a bit of a stretch; it doesn't explain why there
is no trace of anything developing in later stages and no one has ever
been able to create laboratory conditions that have allowed new life
to develop. Miller-Urey got as far as amino acids but that is a long
way from a life form.
Just to be clear, I am not endorsing MarkE's arguments; I'm simply challenging the Gould statement and the "all you need" comment.
A point I was thinking of mentioning to Mark Ellington - human intuition
is a poor guide to processes that operate on spatial and temporal
timescale far removed from everyday experience. Not all processes can be replicated on laboratory scale in human timescales. (Try to produce a
star or a volcano in a laboratory.) I think that we are within sight of directed abiogenesis in the laboratory (or have already achieved it if
one considers viruses living), but I have no reason to think that
spontaneous abiogenesis of something comparable to cellular life is
possible on those scales. (There's a report of spontaneous formation of replicating RNAs in a system - but this system includes the complex macromolecule Q-beta replicase, so even if one argues that this is a
case of spontaneous abiogenesis, it is not relevant to the origin of
life on earth.)
People have found abiotic routes to more than just the amino acids
generated by the Miller-Urey experiment.
We don't know whether life arose only once on earth. We don't even know
whether the descendants of only one origin of life are currently present
on earth. Environmental DNA studies have discovered the existence of
divergent bacterial and archaeal clades that we had known nothing about. Microorganisms with a different underlying biochemistry would be even
more difficult to find.
If life did originate more than once early in Earth history, it's is
more than possible that the descendants of some instances would be
outcompeted into extinction, or even just lost due to environmental
change - perhaps the Great Oxidation Event saw them off (personally I
doubt that any survive so long, but see the above comment about human intuition).
I have speculated before that prior to LUCA there were waves of
replacement as lineages added to their genetic codes, and the ones with
more biochemical versatility outcompeted their sister groups. For adding cysteine to the genetic code allows the production of more stable and presumably more effective proteins.
There are a couple of reasons to think that more recent abiogenesis is
not possible - firstly the chemical environment is different, and
secondly there are living organisms that would see any new arrivals on
the block as food. And if those reasons are invalid, and it did arise,
it would lack the billions of years of improvement shared by organisms
with older ancestry and would likely be outcompeted and go extinct. And
even if that wasn't the case the smaller and less diverse a clade is the
more likely it is to go extinct; if abiogenesis occurred in the Jurassic
and its descendants went extinct in the Cretaceous, how would we know?
A couple of parallels
Two types of symbiogenetic organelles are prevalent in living eukaryotes
- mitochondria and their derivatives and plastids and their derivative.
There is a more recent one, the cyanelles of Paulinella, with a
restricted phylogenetic range, and some pre-organellar symbionts. Can we
be confident that the cyanelle-containing clade will not go extinct in a geologically short time period? How do we know that other organelles
didn't arise in lineages that have subsequently gone extinct.
Eukaryotes contain several ancient multicellular lineages (e.g. animals,
kelps and plants, plus however many exist among other algae, fungi and
slime moulds). There is a more recent clade - the volvocids - with a
minimum level of cellular differentiation. Can we be confident that the volvocids will not go extinct in a geologically short time period? How
do we know that other multicellular lineages haven't arisen and
subsequently gone extinct.
--
alias Ernest Major
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