As we will come to realize time and time again throughout this series, a study of zoology is
in many ways a study of evolutionary biology. And as much as evolutionary biology describes changes
in populations of organisms over time, it also inevitably leads to questions regarding origins.
What is the origin of life on Earth in general, and what were the steps that took place
in order to arrive at the first organism that could be regarded an animal? As we
discussed in the introduction to this series, animals are eukaryotic multicellular organisms,
so the emergence of these two characteristics must be discussed in order to truly grasp this subject.
While we are on the topic of origins, we may as well go all the way back to the origin of life
in general. Most people are completely unaware of the incredible work being done in origin of life
research, and while we do not know the precise series of events that occurred on the molecular
level to yield the earliest forms of life, what we do have mapped out is quite astounding, with
more information being uncovered each year. The field itself is interdisciplinary and unimaginably
complex, unifying concepts in astrochemistry, geochemistry, systems chemistry, information
theory, and many other disciplines, in a way that is dramatically beyond the scope of this course.
But we can briefly and generally say that small molecules which were abundant on the early earth
reacted to form simple biomolecules, which then polymerized to form biomacromolecules,
which were encapsulated in spontaneously forming lipid membranes. Within these, a wide variety of
autocatalytic reactions ensued, in which molecules catalyzed the formation of other molecules,
until certain systems of molecules stumbled upon the capacity for self-replication by chance,
at which point a kind of selection and evolutionary pressure took hold, whereby systems
of molecules with certain properties predominated statistically, and over hundreds of millions
of years, some kind of rudimentary protocells emerged, capable of reproduction and metabolism.
I have some other content that goes into these concepts in considerable depth which I’ll link
to in the description for those who are eager to dive into the primary scientific literature,
but for everyone else let’s just start here, with some very simple unicellular prokaryotic life. In
here we have some genetic material, and lots of proteins performing various cellular functions.
What happened next? Well these cells continued to evolve over millions of years,
producing new and varied proteins, and sometimes stumbling onto brand new capabilities by chance,
such as more sophisticated methods of cellular respiration as well as photosynthesis. Then a
remarkable event took place, which is described by endosymbiotic theory. As we discussed in the
biology series when describing the emergence of eukaryotic life, at some point, larger prokaryotic
organisms became able to engulf other prokaryotic organisms without ingesting them, and a mutual
dependency arose between the smaller one, which receives shelter, and the larger one, which reaps
the benefits of these novel abilities possessed by the other organism. This is where we get the
term endosymbiotic, as endo means within, and symbiosis refers to a cooperative relationship.
As these systems of organisms evolved, the smaller ones became more and more specialized,
until they eventually became fundamental components of the larger organism,
such as chloroplasts and mitochondria. This is how eukaryotic organisms came to be,
from an infolding of the cell membrane to generate a nuclear membrane and several organelles,
as well as yet other organelles arising due to endosymbiosis.
We learned all about these various organelles and their functions in the biology series, so we don’t
need to review the features of eukaryotic cells here. But that does leave us with one more very
important leap to address. Eukaryotic unicellular organisms, like all the protists that exist even
today, are much more complex than prokaryotic life. But more complex still, animals are
multicellular. So how did the first multicellular organisms arise around 600 million years ago?
This was also discussed briefly in one of the biology tutorials, where we mentioned the
interesting properties of choanoflagellates. These are unicellular protists, so they are eukaryotic.
And they are remarkably similar to collar cells on certain sponges. Given this and other data,
biologists believe that unicellular organisms like this are the ancestors of all animal life.
The jump from unicellular to multicellular is described by the colonial flagellate hypothesis,
whereby many organisms were able to aggregate to form simple structures, after which a kind
of specialization began to take place, as the result of methods of interorganismal communication
that strongly resemble methods of intercellular communication that occur in animals today.
Chemical signals can have impacted gene expression for specific cells in specific ways, resulting in
subsequent infolding and the production of more significant structural features.
The driving force for this process in an evolutionary context is just the same as
what we saw for the emergence of eukaryotic life. Where there was a symbiosis between
prokaryotic organisms that eventually led to eukaryotes, there must have been
a symbiosis between unicellular organisms that eventually led to multicellular life.
The structure and differentiation of function exhibited by the colony of cells afforded a
greater probability of survival to the group, and the concept of cell specialization was born.
These early multicellular organisms provided the origin of kingdom animalia,
and it is the case that choanoflagellates are the closest relatives of the animals. Not only do they
resemble sponge collar cells, as we mentioned, but they can also reproduce sexually, and even
use the same proteins for cell communication and adhesion as animal cells. In addition, numerous
independent studies of their 18S rDNA gene, nuclear protein-coding genes, and mitochondrial
genomes, all place the choanoflagellates as a sister group to the animals, or metazoans.
In fact, some choanoflagellates even self-organize into colonies that feed by creating currents
somewhat like sponges. So as we can see, there are multiple threads of evidence that
corroborate this idea regarding the emergence of multicellular life, and animal life in particular.
And with that, we have briefly discussed transitions from simple biomolecules
to prokaryotic cells to eukaryotic cells to multicellular organisms to the kingdom animalia.
Each of these transitions could be expanded upon in extreme detail,
but given that zoology is the study of animals, let’s continue forward from
the emergence of animals and begin to learn about animal development.