Alien Biosphere Evolution #6: Size and the Modularity of Life - Lake Harding Association

Alien Biosphere Evolution #6: Size and the Modularity of Life

By Micah Moen 0 Comment March 26, 2020


When it comes to the evolution of life on
Earth or other planets the general rule is that:
Probabilities decrease with increasing specificity. The more specific a life form, the less likely
it is that it will evolve naturally. A humanoid being, for instance, though often
the staple of science fiction, may actually be relatively unlikely, because of the long
chain of contingent events needed to arrive at it. That doesn’t mean anything more specific
is implausible, but just that we can expect it to be less common in the universe. No matter what, we can be certain about one
thing: Life will start out tiny. From mere molecules to minute microbes, life
on any exoplanet far, far away, will probably be stuck at microscopic scales at first and
remain so for aeons. That is because, starting from such humble
beginnings, evolving creatures need to achieve key innovations in order to break through
into the macroscopic world. So what are some of the strategies employed
by living systems during evolution to attain ever greater sizes and is there a single universal
trend? Let’s find out! Any planetary biospheres out in the universe
will expectedly go through a natural development in which there will be predictable and unpredictable
events. When doing speculative biology, the challenge
is to sort out which are predictable and thus bound to happen and which are not. And the more specific a particular creature
design, the less likely it may be for it to appear again elsewhere, because of all the
conditions required for it occuring in the right order. In contrast, the eventual genesis of single-celled
organisms out of proto-biological chemistry is much more predictable. And this is why: All life will initially start out on the tiniest
scale conceivable: The molecular level. And for any self-replicating molecule, gathering
enough chemical components needed for the assembly of another copy of itself is -in
principle- just a matter of time. However, the passage of time also increases
the chances of its destruction. So to speed up matters, a self-replicator
may want to start producing other molecules like enzymes, to act as little minions to
aid it in its quest to take over the world. But it would be nice if everyone stayed together. So the adoption of a form of container around
all of this taking care of just that would be advantageous and give an edge over the
competition. Life on Earth co-opted a special kind of naturally
occurring lipids that can form membranes as containers for metabolic processes. In time, a dependency developed on this cellular
structure achieving something we could call a single-celled organism or microbe. And it is highly likely that life elsewhere
will do the same or something very similar. But there are very obvious restrictions to
size for microbes. Just increasing cell volume defeats the whole
purpose of having a membrane in the first place, as the concentrations become too low
for metabolic processes to be effective. Also, nutrient uptake from the surroundings
will suffer due to the growing surface-to-volume ratio. So life will always start out microscopically
and stay small for quite a long time, long enough for establishing dominance of any early
biospheres. Single-celled life will monopolize resources
and microbial mats will trap vital minerals in the underlying substrate as well as diminish
oxygen levels above and below. This will probably universally result in what
I call Microbial Phase biospheres. For a long time, biospheres will remain in
a sort of stalemate. The paradox is that, in order for active macrofauna
to evolve in the first place, you need active macrofauna with adaptations for working through
sediments. So how can primordial life forms increase
their relative size and get the revolution started? Well, first of all, by using or rather re-using,
what they’ve already got: Membranes. Membranes can be folded inwards as to compartmentalise
different metabolic processes. On Earth, these developments made microbial
cells ever more complex and eventually led to what we call the Eukaryotic cell: The building
block of plants, fungi and animals. For instance, to make enzyme production more
effective a microbe’s genetic material could become enveloped by a special internal membrane
thus forming a nucleus. Other important processes to be compartmentalised
are those related to energy-production. This is where interesting developments took
place early in the evolution of eukaryotes: The assimilation of other microbes to act
as a kind of household servant. Oxygen-burning microbes were turned into the
internal power plants known as mitochondria. Photosynthesizing microbes became the chloroplasts
inside the cells of plants and algae. This phenomenon is called endosymbiosis and
essentially resulted in cells within cells. Transport of materials in these super-cells
is taken care of by a network of membranes called the Endoplasmic Reticulum combined
with a protein scaffolding formed by so-called microtubules. This system enables more control over what
goes where rather than relying on mere diffusion. And with many cellular processes having become
bound to membranes in the first place, the greater surface area will then also lead to
increased production and wider distribution. All this complex machinery allows for cells
that can grow substantially bigger. In another video, I will speculate on other
ways to achieve the same kind of complex cell, but for now, let’s move on to the next level. Because not even a eukaryotic cell can become
much larger without additional techniques. One obvious way is simply duplicating and
spreading vital structures into a larger volume known as a coenocyte. Macroalgae like Caulerpa or Valonia and plasmodial
slime molds are examples of such creatures with a continuous, multinucleate cell. But there’s a better way to outgrow unicellular
mediocrity that also offers greater flexibility. As a complex, self-contained module, the entire
cell itself can be duplicated and re-used. And in everything from biology to human technology,
modularity is what enables ever higher levels of complexity. I’m talking -of course- about: Multicellularity. Duplicating the same complex structure over
and over again, while varying, specializing and arranging behaviour and properties, can
obviously lead to many innovative solutions. This especially goes for microbial cells and
multicellularity has evolved many times over in different lineages of life, perhaps starting
as simple colonies of cells clumping together. By secreting fiber-forming molecules, cells
could be glued together into what is called an extracellular matrix which literally means:
a lattice or fabric outside of the cells. However, cells need a couple more key adaptations
to really get up and running. Like how the lego brick caused a toy revolution
with their interlocking knobs, the microbial ancestors of animals had to
evolve specific molecular connections in order to coalesce into cohesive units or
tissues. And once cells became able to form tissues,
this opened for a wealth of new possibilities. When these cellular aggregates learned to
form a cohesive mat by dividing mostly laterally, while resting on a dense matrix, they formed
a special kind of tissue layer known as an epithelia. A tissue like that is essentially a membrane
of a higher order that can fold around and thus contain a fluid-filled cavity in its
own turn. The immediate advantage of this would be a
dramatic increase in size compared to merely a tight clump of cells, not unlike these Volvox
algae. This is quite effective as a form for protection
against predators with a minimum number of cells needed, all while new colonies can be
formed in the safe space inside. An interesting parallel to this is the defensive
circle that Muskoxen create around their young. But it doesn’t stop there: A creature consisting
of cohesive tissues has the capacity to keep on growing while retaining overall shape and
properties, like a balloon being inflated. You can compare it a bit with those gummy
bear candies, which will swell by drawing in water, while preserving form, colour and
taste to some degree. In a similar way, a multicellular blob can
simply grow by increasing its internal volume while adding more cells on the outside, both
laterally but also out- or inwards, if need be. The next step is specializing different areas
of the tissue layer to suit various functions. Single-celled organisms often already have
diverse cell types at their disposal manifested during different phases of their life-cycle. For instance, the Percolozoa have an amoeba-phase,
when food is plenty, a flagellate phase for migrations and secrete tough outer walls,
while staying dormant during harsh times. These cell phases can be marshalled as distinct
cell types in multicellular creatures. Some cell types could provide movement while
others work on digesting any food encountered. A third cell type could migrate inwards to
provide internal structure by secreting tough materials. Animals with this really simple configuration
actually exist and they’re called Placozoans. At this stage, tissues have become modules
in their own right, but do need some kind of polarity for determining which way is up,
down, or perhaps even forward. Groups of certain types of cells can also
start to fold inwards creating internal organs like a digestive cavity or gut. Or they can fold outwards making external
body parts like tentacles, gills or limbs. Different tissue types can come together forming
whole organs, giving our evolving creatures yet another modular level for even greater
possibilities. But just like with the single cell, above
certain sizes creatures start running into troubles again and will need organs specialized
in respiration and circulation of vital substances. As higher level modules, organs themselves
can be made to appear at a specific spot on the body or even be duplicated at multiple
spots. Organs can also be organised in organ systems,
yet another level of module, and when laid out in repeated serial patterns the body can
be extended into even greater sizes. This phenomenon is called metamerism and lies
at the foundation of the segmentation seen in arthropods & annelids as well as the series
of vertebrae and ribs in our own bodies. So thinking about the modularity of living
systems really reveals its fractal nature with repeated elements being organized by
nesting variations into ever larger frameworks. Starting from the mere need for a membrane
to make duplication more efficient: Different kinds of membranes enabled larger
and more complex cells. Complex cells were then grouped together in
specific ways in order to form tissues. Various tissues came together to form complex
organs. And organs can be gathered into organ systems
and body parts which can then be repeated in various patterns to create complicated
body layouts. In the next video, I will discuss deep body
patterning and the enormous possibilities we’ll then have at our disposal for designing
naturally evolved creatures. For now, Please leave a comment. Don’t forget to like, subscribe and click
the bell icon! Cheers and bye, bye!

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