The Philosophy of Sex, Part II: From Bacteria to
Complex Life Forms
By Josh Glazer
It
has been argued in the past, most notably by Richard Dawkins, et. al., that the
most basic unit of selection, at the evolutionary level, is the gene. In it, we hit a kind of biological bedrock,
in which it is difficult to move beyond.
We know that certain genes contain the instructions (in DNA format) for
creating specific proteins, the basic building blocks of life. But we don’t know how DNA ‘evolved’. We know how the properties of certain
proteins emerge; from their specific makeup of amino acids, and their 3-dimensional
structure, for example. We also know
that there are long stretches of so-called ‘junk’ DNA that do not code for any
effective proteins . Some of this junk
DNA is the debris from genes that were lost in the evolutionary past. Yet some junk DNA holds vitally important switches
which control key components of embryonic development, such as the timing and
activation of certain body parts in their growth cycle. There are still large gaps in our knowledge
of how DNA came about, and the earliest life, but we are beginning to make
inroads towards an understanding of how both simple, and complex, life forms
can develop, and even evolve, from certain sequences of genes.
We do have rich archeological
and geological records, among others, and
along with modern genomics, we are able to extrapolate a great deal of data
about the different lineages of animals, and also their lateral transfers of
genes, which can happen at the bacterial level, as will be discussed
below. What we want to explore, here,
however, is more basic question: how did complex body plans form from simple
multi-cellular organisms?
In
order to begin our inquiry, it will be helpful to briefly review, at this point,
the first 4 billion years or so of earth’s history, and how life behaved during
that time. The earth formed as a fiery
pit of asteroid collisions and volcanic eruptions about 4.5 million years ago,
about the same age as the solar system as a whole. After a period of cooling and a cessation of
tectonic fireworks, the first vestiges of microbial life have been established
as about 3.5 million years old. These
consist in faint impressions of the first living cells on this planet. (Prothero, 164)
These
first cells, however, were simple in nature, lacking nuclei, and other complex
components of life as we know it now.
These simple cells, or bacteria, would be the only life forms on the
planet for about 3 billion years, until the formation of the first complex cell
with a nucleus – the eukaryotic cell (Latin for ‘having a nucleus. Prokaryotes are cells without a nucleus). What brought about this transformation? It is generally agreed upon now, although it
was quite controversial when first published, that the eukaryotic cell was the
result of two or more bacterium combining to live in symbiosis with one
another. In this way, the waste products
of one bacteria could be used as the fuel for another type of bacterium, and
vice versa. Although prokaryotes far outnumber
complex organisms on this planet, all plants and animals are made up of
eukaryotic cells.
The
question now becomes; how did life make the jump from eukaryotic cells to complex
life, with body plans that continued to evolve?
We can begin our inquiry with the first possible ‘version’ of sex, known
as ‘bacterial conjugation’. This, simply
put, is when two bacteria link up via a cytoplasmic tube, and freely exchange
genes. Exactly why they do this, we do
not know. Another form of bacterial
genetic exchange is the existence of ‘plasmids’, which are basically just
floating chunks of bacterial DNA, ready to be taken up by any bacteria in the immediate
area. These methods are still in use
today, as, for example, bacteria pass genes for resistance to modern
antibiotics between one another, leading to an arms race between the bacteria’s
ability to evolve counter-measures, and our ability to devise efficient
anti-microbial medicines.
But
even these forms of ‘bacterial sex’, or, more literally, the exchange of DNA
between two bacteria, are not representative of sexual reproduction, as it is
known in more complex life. A bacteria
divides by asexual reproduction, making an exact copy of itself. Rare mutations may occur through radiation,
copying errors, or UV light, but these mutations are not nearly enough to
produce the millions of species that exist on the planet today.
In
order for more complex body plans to exist, sexual reproduction had to
evolve. This came about around the
beginning of the Cambrian Explosion, as it is known, about 650 million years
ago, when radiations of new body plans and forms began to proliferate. Why did this new form of reproduction lead to
such a vast new number of organisms? As
opposed to asexual reproduction, sexual reproduction is a complex procedure
which thoroughly mixes the genes of the two parents. To summarize:
“Meiosis is simply the procedure
by the which the male selects the genes that will go into a sperm or the female
selects the genes that will go into an egg… During meiosis something peculiar
happens. Each of the 23 pairs of
chromosomes is laid alongside its opposite number. Chunks of one set are swapped with chunks of
the other in a procedure called ‘recombination’. One whole set is then passed on to the
offspring, to be married with a set from the other parents – a procedure known
as ‘outcrossing’...
“Sex is recombination plus
outcrossing; this mixing of genes is its principal feature. The baby gets a thorough mixture of its four
grandparents’ genes (through recombination) and its 2 parents genes (through
outcrossing).” (Ridley, 29-30)
This
blending of genes eventually leads to the creation of an embryo, which then
develops into a more fully grown young, ready to be exposed to the world. One question that may still be asked at this
point is, how does all of this ‘genetic mixing’ lead to the incredible
proliferation of life forms in the Cambrian period, and afterwards?
The
answer may lie in a relatively new biological field call ‘Evolutionary
Developmental Theory’, or just ‘Evo / Devo’ for short. This new field lies at the crossroads of
embryology and evolutionary biology; hence the name. (Carroll, 9)
Essentially, it compares an embryo’s development across several related
species, and tries to figure out what controls the formation of the body,
limbs, etc. Being that 98% of our DNA is
the same as chimpanzees, that 2% is rather important in the understanding of our
human species. Evo / Devo seeks to
understand how embryos both develop and evolve by creating genetic maps, which
indicate where certain body parts begin to emerge from the once single celled
embryo.
The
first studies of Evo / Devo were done using simple laboratory animals like
fruit flies, and provided almost miraculous results. They revealed, after years of studies on the
fruit fly’s third chromosome, that there sat 8 genes, in order, which formed
the entire body of the fruit fly. More
so, all animals, from fruit flies to mice to humans, shared a similar set of
these form making genes. They controlled
modular formation of the organism, such as you might see in a millipede; the
same structure repeated a thousand times.
Or, a new appendage grown where there had been only a bud before. Through careful manipulation, scientists were
able to show that these genes existed across major species lines; growing weird
mutants with legs in strange places, all to demonstrate the ubiquity of these
genes. These genes were somehow
controlling the growth of the embryo, telling it what to do and when to do it,
in order to create a specific organism.
Since
the form of these genes was similar across protein domains (existing spaces of
possible DNA coded proteins), ‘The shared DNA sequence was dubbed the homebox
and the corresponding domain it encoded, the homeodomain… Hox genes for short.’
(Carroll, 65) The discovery of
these Hox genes opened a whole new window into the way that body plans were
organized through genetic mixing; simple switches could enable a particular
body part to either multiply or appear at a different place. Over evolutionary time, this was enough
evidence to explain the immense diversity of organisms on this planet, and also
their basic similarities at the same time, like DNA and other formations, such
as Mitochondria and other organelles.
One incredible revelation is that all the instructions for most animals
are contained in the DNA of similar animals; it is the genetic switches (or Hox
genes) which controlled what life form would develop from the initial blastula.
(Above
Picture, from Carroll, 62. Illustrates
Hox genes expressed in a fruit fly.)
And so, we can see that the
genetic mixing of sexual reproduction greatly increases the chances of new
adaptations, which, over evolutionary time, can create entirely different
forms, based on an organism’s adaptation to its environment. Additionally, when we use all of our most
advanced scientific techniques to look backward in time towards our most
universal common ancestor, it appears that many of the key elements of life
already existed almost 1 billion years ago; it was merely a matter of arranging
them in the correct format.
Here,
we will end Part II. In Part III, we
will finally see how sexual selection in human beings has helped shaped human
nature, our species, our history, and even our different cultures.
Sources Cited:
Carroll, Sean B. Endless
Forms Most Beautiful: The New Science of Evo Devo. Norton, New York (2005).
Jastrow, Robert & Rampino,
Michael. Origins of Life in the Universe. Cambridge, London (2008).
Ridley, Matt. The Red Queen:
Sex and the Evolution of Human Nature. Harper Perennial, London (2003).
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