John O. Campbell
All life comes in the form of cells. This
immediately raises the question: why? Why should all life have this unique
design in common? It might seem plausible that the evolution from chemistry to
life would have explored many pathways not involving a cellular architecture
whose descendants should be observed to this day. It seems plausible that we
should observe a near continuity of strategies for achieving life, yet we
observe only a single fundamental, highly evolved, strategy for life.
This simple but surprising fact implies
that proto-life evolved in a specialized environment over a long period of
time. Only after evolving many fitness-conferring adaptations in that
environment did it emerge and move on to colonize almost the entire planet. The
long list of sophisticated adaptations shared by all life implies a universal
common ancestor which possessed those adaptations. It implies that all
subsequent forms of life inherited their cellular architecture and this long list of
common biochemical adaptations in a direct line of descent from this last
universal common ancestor (LUCA).
Indeed, this reasoning, has led to a near
consensus that all existing life has descended from a ‘last universal common
ancestor’ or LUCA. It is posited that this distant ancestor possessed the full
list of life’s characteristics and all subsequent life has evolved from LUCA
and retains those fundamental characteristics (1).
As the good regulator theorem reminds us,
any complex system, such as LUCA, forms a model of the system it is regulating (2). This simple
understanding provides insights that may help to identify the particular niche
in which LUCA evolved; likely niches are those which reflect or mirror the
architecture of LUCA and which provide a suitable environment for the evolution
of its many adaptations.
‘Origins of life is’ an extremely active
area of research and many scenarios have been considered. Among these,
Wikipedia suggests alkaline oceanic vents as perhaps the most likely niche in
which LUCA evolved from chemistry (1):
The cell probably lived in conditions found in deep sea vents caused
by ocean water interacting with magma beneath the ocean floor.
Indeed, the hydrothermal vent hypothesis is
perhaps nearing consensus among current research as the most likely origin of
life scenario (3). Essentially this
hypothesis notes that porous rocks lining the boundary of alkaline hydrothermal
vents provides cell-like niches supporting many pre-biotic chemical
requirements. These include an energy gradient across pore walls due to
alkaline vent flows on one side and more ambient ocean conditions on the other.
Such rocky cells provide a rich niche where the chemical precursors of life
could become more concentrated than in other more typical oceanic environments (4).
A most surprising thing about LUCA is the
advanced state of its evolutionary development. LUCA was a not a simple
chemical-like mechanism composing only a few rudimentary adaptations. Rather it
was a sophisticated bundle of adaptation fine-tuned to its rocky-vent habitat.
This implies that a long period of evolution occurred within the hydrothermal
vent environment leading to LUCA and that life evolved beyond LUCA only when it
finally inferred how to exist in the wider oceanic environment.
We should understand that LUCA was
extremely well adapted for life within the hydrothermal vent niche. We can
describe this adaptation both in Darwinian terms and in terms of the free
energy principle. The FEP perspective asserts that LUCA had developed a genetic
model of a survival strategy in this niche and that it acted to carry out this
strategy as accurately as possible. Life before LUCA evolved to produce
adaptations that could take advantage of the potential for existence offered by
the hydrothermal vent niche. In a sense LUCA and the rocky pores of
hydrothermal vents became mirrors of each other minimizing free energy.
Undoubtedly, as LUCA more fully filled this
niche, additional resources and suitable niches became scarce and new forms
evolved to make use of a wider range including marginal resources and niches.
There is little existing evidence of these experimental forms other than the
two forms which ultimately colonized the wider oceanic environment and became
the ancestral forms of all life beyond LUCA: bacteria and archaea, more
technically named eubacteria and archaebacteria.
These two forms differ somewhat from LUCA.
The greatest differences might be summarized as their possession of a cell wall
and possessing a new type of chemical machinery for duplicating their genomes.
In both instances the detailed chemical machinery, including all the genetically produced enzymes
involved, are quite different in bacteria and archaea – strongly suggesting
that these adaptations where achieved through independent evolutionary
trajectories.
These significant differences should not
obscure their otherwise overwhelming similarity. Both bacteria and archaea
share all the trademark characteristics inherited from LUCA. Specifically,
their morphology is almost the same; under the microscope they look identical
and may be identified only through sophisticated biochemical analysis.
Crucially, their overwhelming similarity is in their overall architecture –
their cellular structure.
An obvious evolutionary source of this
similarity is LUCA’s adaptations to its niche in pores of the rocks forming the
boundaries of hydrothermal vents.
Figure
9: Taken from (5).
A 360 Myr old hydrothermally formed iron sulphide chimney from Silvermines,
Ireland, (Boyce et al. 1983). Pores in this rock form chemically active
compartments with dimensions comparable to biological cells.
As recent research indicates (5; 6), bacteria and
archaea are likely to have become free living organisms in the wider oceanic
environment by independently reconstructing their cellular architecture through
the evolution of cell walls:
All life is organized as cells…
The naturally arising,
three-dimensional compartmentation observed within fossilized seepage-site
metal sulphide precipitates indicates that these inorganic compartments were
the precursors of cell walls and membranes found in free-living prokaryotes.
Indeed,
detailed explanations of plausible evolutionary routes leading from LUCA to
bacteria and archaea have been suggested whose main feature is the evolution of
cell walls able to provide a chemical habitat closely mimicking the pores of
hydrothermal vents to which LUCA was well adapted.
Figure 10: From (5) A model for the origin
of membrane-bounded prokaryotic cells from iron monosulphide compartments
within which the chemoautotrophic origin of life could have occurred.
At this point we might step back a little
and consider an important implication of this scenario. LUCA was probably the
product of several hundred million years of biochemical evolution taking place
within the rocky pores of hydrothermal vents and all its adaptations were to
this one niche. Obviously, the wider ocean environment offered a good deal of
potential for any variant of LUCA able to exist there but how could existence
be achieved when all LUCA’s adaptations were to a cellular niche formed by rock?
Bacteria and archaea both inferred the same
successful strategy: reconstruct cellular niches through the invention and
construction of cell walls. This is an example of the well-known biological
mechanism of niche construction (7) where organisms,
such as beavers, evolve the ability to construct niches through mechanisms such
as building dams, niches to which their non-dam-building ancestors were already
adapted.
At the core of biological evolution is the
increased fitness of organisms to their environment or in terms of the FEP,
the minimization of genetic prediction error. Niche construction
represents a dual to this usual understanding by noting that fitness may also
be increased if the organism acts to alter its niche to better fit to its existing adaptations.
References
1. Wikipedia. Last universal
common ancestor. Wikipedia. [Online] [Cited: June 9, 2018.] https://en.wikipedia.org/wiki/Last_universal_common_ancestor.
2. Every
good regulator of a system must be a model of that system. Conant, RC
and Ashby, RW. s.l. : Int. J. Systems Sci., 1970, Int. J. Systems
Sci., pp. 89–97.
3. Lane,
Nick. The vital question. s.l. : Profile Books, 2015.
4. Acetyl
Phosphate as a Primordial Energy Currency. Whicher, Alexandra , et al.
s.l. : Orig Life Evol Biosph, 2018.
https://doi.org/10.1007/s11084-018-9555-8.
5. On
the origins of cells: a hypothesis for the evolutionary transitions from
abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to
nucleated cells. Martin, William and Russell, Michael J.
s.l. : Philosophical transactions of the Royal Society of London. Series
B, Biological sciences, 2002, Vol. 258 1429.
6. Early
bioenergetic evolution. Sousa FL, Thiergart T, Landan G, Nelson-Sathi
S, Pereira AC, Allen JF, Martin WF. s.l. : Phil Trans R Soc B, 2013,
Vol. 368: 20130088. http://dx.doi.org/10.1098/rstb.2013.0088.
7. A
variational approach to niche construction. Constant, Axel, et al.
s.l. : Journal of the Royal Society Interface, 2018.
