Until
the middle of the nineteenth century, before the publication of the Origin of
Species, by Charles Darwin, it was generally believed that all kinds of animals,
plants and microorganisms had somehow arisen independently from one another.
For most of humanity, the Biblical Genesis sufficed as an explanation. God
created every living species, including ours. There were even attempts to
calculate the exact date of creation, by adding the ages of some characters
mentioned in the scriptures. The conclusion was that everything was created
approximately six thousand years ago.
Philosophers such as Sir William Paley
also presented additional evidence of the truth of creation. Using analogy, he
reasoned that when we see a complex object, like a watch, we presuppose the
existence of a watchmaker, since that object could never have formed by chance.
The even grander complexity of nature and of living beings also indicated the
existence of a plan or project, of an extremely intricate creation.
But the power of an argument can
sometimes lead us astray. Humanity also used to regard the Earth as the static
center of the universe. There are facts that are at least strange with the idea
of creation. For instance, the number of species on Earth, not counting
“simple” organisms such as bacteria or archaea, is estimated to be around
twenty million. But less than two million species had already been officially
known and catalogued, and about half of those are animals. Among the catalogued
animals, more than a third are beetles, and the proportion is probably larger
than this. If God really created every species, then He has an extreme fondness
of beetles!
What is more, very often during
excavations fossilized bones were found, and those bones apparently were
totally unlike those of any known living animal. This was seen as evidence of
the great flood, but there was a problem: the kinds of bones changed the deeper
the dig was, indicating many floods. That was not in the bible. There was
another somewhat ad hoc explanation,
according to which God had created the world with all those bones buried, to
give us the impression that it was older than it really was. As Einstein once
remarked, subtle is the Lord, but malicious he is not! There is a scientific
principle called Occam’s razor: when there are two explanations for the same
phenomenon, it is wise to adopt to simplest one.
How, then, can we explain the diversity
of life and all its complexity? How to make sense of the existence of fossils?
Inspired by some ideas of the reverend Thomas Malthus and observing nature,
Darwin found a simple enough answer: species changed with time, or evolved. As
we will see, this idea is so profound we can even say that Darwin did not
invent it; he discovered it.
Malthus had posited that the human
population tends to grow much more rapidly than the quantity of resources
necessary to sustain it. That is, resources are scarce and don’t allow a
population to grow indefinitely. It’s easy to understand why: if every
individual in a population fathers four offspring, than any specific individual
will have four sons, sixteen grandsons, thirty-two great-grandsons, then sixty
four great-great-grandsons and so on, in a way that after a few generations his
descendants would fill the whole surface of this planet, and there wouldn’t be
any place left for anything else. Clearly, population stops growing well before
this extreme situation materializes.
If we tend to reproduce quickly, but
there are not enough resources for everyone, the consequence is what we already
know: it’s hard to make a living; we have to struggle to survive. We compete
for resources. When there are plenty, our population grows. That is what is happening
today, due to the affluence brought by industrial society. But when we become
too numerous, widespread famine ensues. Starvation debilitates us, and
epidemics of various sorts follow. In this way our numbers gradually diminish.
Thanks to human sophistication there is always a third way to resolve
overpopulation: warfare.
In a population no two individuals are
exactly alike. Some people are taller, others are more resistant to heat, some
have a darker skin, and so on. This difference means that some will have more
access to resources than others. In general, those who get more resources also
survive longer and leave more offspring. On the other hand, some individuals
are not able to gather enough resources, and die before they successfully
reproduce. The ones who end up with more descendants have traits that confer
them with an advantage in relation to others and make them better adapted to
the environment in which they live. These same traits are passed on, while the
ill-adapted individuals almost never transmit their features.
Darwin
saw, as Malthus before him did, that this reasoning was valid to any species.
And he went further: different places, like a desert and a mountain summit, put
different strains on a given population. In each place, the favored traits will
be different. When a population that formerly lived together separates in a way
that each part migrates to different and isolated habitats, the two newly
formed populations start to adapt to their environments separately, transforming
themselves beyond recognition after a long time.
It’s not just the physical conditions
that determine which individuals are better adapted to a specific place. Other
living beings are also part of the environment and must adapt to each other.
Thus, the ideal characteristics are not always the same; they are constantly
changing. The leopard needs to run really fast to capture gazelles, and these
need to outrun the leopards to survive. Slow leopards get fewer meals and
become weak, eventually dying without offspring. Accordingly, slow paced
gazelles rarely pass their traits on to the next generation.
Darwin did not know what laws governed
heredity, though they were published by Gregor Mendel soon after the
publication of the Origin. His ignorance weakened somewhat his arguments. His
favored theory of heredity involved the mixing of paternal and maternal
features, something that would dilute the differences between individuals and
create a homogeneous population.
The discovery in the twentieth century
of the structure and function of the DNA molecule completely confirmed the
assumptions of evolutionary theory. The DNA present in our cells contains
instructions for the production of proteins that determine all our physiology
and morphology. All this is done using a digital code similar to the one
employed by computers, only with four letters, A,T,C,G, instead of the two –
0,1 – used in electronic devices. In its long sequence there is always an
instruction to build copies of itself. DNA molecules unite in “teams” that may
gain an advantage over other teams. Each team member is called a gene. A gene
can be seen as one of many varieties of a given feature, such as eye color. The
better the combination (or team) of genes inherited from its parents, the more
adapted an individual is to a certain environment, and the better its genes
will succeed in making copies of themselves. Genes instruct: “build a giraffe
with such and such features”. If the resultant features are better at getting
the giraffe more descendants, the team of genes does not die with the giraffe,
and is passed on.
The features belonging to each parent
do not dilute because they are received in ‘packets’ by their offspring. Our
parents randomly transmit to us half their genes. For each of our distinguished
characteristics, we own two genes – one given by our mother and the other by
our father. But in general, only one of them prevails. Diversity is generated
in nature because of this shuffling of genes brought about by sexual
reproduction and also due to rare copying errors in the DNA molecule, called
mutations.
The name evolution may give an idea of
progress, from simpler forms of life to more complex ones. This is an issue
still debated, as the idea of increased complexity being adaptive, especially when
other species are part of the environment, is not unreasonable. But this
‘progress’ doesn’t mean a specific direction toward any objective, much less
teleology. Natural selection is a blind watchmaker, in the words of biologist
Richard Dawkins. In fact, when life originated, all species were simple and
microscopic. Later, many complex species arose, but these form only a small
percentage of all living species, whose numbers increased astonishingly. The
kingdoms of bacteria and archaea continue to be the vastest, with perhaps
ninety percent of all living species.
Some objections were raised against the
theory of evolution, in part because it was misunderstood. If evolution is
blind and random, how organs so complex as the eyes would come up? And what about
complex beings? If this were possible, wouldn’t it also be possible for a
hurricane to strike a junkyard and build a jet airliner? What’s more, if
evolution is gradual, taking advantage of slight differences among individuals,
what use would be half eye? Or half wing, for that matter? The answer is that
although environmental changes are capricious and mutations also happen
randomly, selection itself is non-random. Of the numerous possible (and random)
mutations that can happen, only a small number are selected and allow an
individual to bear offspring. At any one time, there are sets of viable and
unviable individuals (who are dead). Among those randomly varied viable ones,
only a few will survive to pass on their characteristics.
So the answer to the objections is: not
only an organ as complex as the eye evolved by random mutations, but it has
also evolved independently about forty times! That is, our eyes are different
from insects’ eyes, which in turn are different from those of squids, which are
different from crustaceans’ eyes, and so on. Dogs, flies, octopi and crabs
evolved their eyes independently from a possible common ancestor with very
rudimentary light sensitive cells. It is even possible that there was no need
for such common ancestor, the process happening entirely independently! Eye
structure is completely different among different orders of beings. How can
this be?
There is a simple explanation, be it
for the eye case or the wing case. For starters, the idea that half eye or half
wing is no use is wrong. In a kingdom of blind, he who has one eye is the king.
In a population in which all members are blind, any individual capable of
differentiating light from shade has an enormous advantage over the others. If
an individual carries a mutation that makes some cells on its body react with
light, it may notice the presence of a predator before the others and save its
life. Or else it may notice that a prey is coming. In no time, its descendants
will be everywhere. If one of these descendants is a mutant that has its light
sensitive cells located on a curved surface or part of its body, then it will
be able to perceive direction, be it of prey or predator, for their images will
strike one side of the surface more than the other. Again, that is a great
advantage, and very soon its descendants would dominate the population. A
further mutation might make this curvature stronger, allowing the image to be
focused, as in a camera. Other mutations might allow color perception, and so
on. It doesn’t sound like an impossible feat at all, if we bear in mind that
all that is needed is random and gradual mutation. Most mutations will actually
be harmful, but some of them will be the ones that will be around at the right
moment when nature is selecting them.
Since mutations are not directed toward
an objective, being random instead, most of them reduce, not increase, an
individual’s fitness. But as different environments impose different
challenges, the concept of deficiency, or disease, may change from place to
place. When a mutation makes an animal myopic, its capacity to spot prey or
predator is diminished. Blindness is worse, making the unlucky victim an easy
prey. If the blind animal happens to live underground, though, blindness ceases
to be a disease, as it couldn’t matter less. To get rid of its eyes can even be
an advantage, because organs are expensive and the energy used to build and
maintain an eye can be spent elsewhere. To a terrestrial animal, to have no
legs is normally fatal, but if its lifestyle includes diving in a lagoon
nearby, this can even make locomotion underwater easier. That’s what probably
happened, albeit gradually, with whales.
Our species is the first on Earth to try
to “stop” natural selection. We protect those we love when they have health
problems or difficulties in being completely independent. In doing this, we
allow mutations that would be inviable in nature (by causing premature death)
to spread, as many who bear them reach reproductive age. Even something as
common today as short sightedness wouldn’t be allowed in nature. Myopic
individuals needn’t worry about fleeing leopards or catching mammoths anymore.
They wear eyeglasses. This generates some more serious problems. Not so long
ago, many hemophiliacs wouldn’t reach maturity to leave descendants. Because
today treatment for this disease has advanced considerably, genes for
hemophilia will no doubt gradually spread in the human population. There are
many other examples. Today it’s even possible for sterile couples to have
babies, which means that genes for sterility will spread and in the future most
human reproduction might need medical assistance. We are struggling - with
technology - to outrun time and nature, aiming to eradicate suffering once and
for all, altering the very genes that cause them. However, this is a mined
field full of slippery slopes, because ethics must come to the stage when we
ponder our notions of ‘defect’. How are to determine what the “good” genes are?
Common sense or “reason” varies. There are those who think certain skin colors
are bad and should be eliminated. Unfortunately our morals don’t seem to evolve
as quickly as technology does. It’s still an open question whether we will be
able to act wisely.
Many people are opposed to evolution,
as it displaced us from a privileged position in creation and made us just one
more species among millions equally successful. Like any other species, one day
we will be extinct. Even if we manage to leave descendants, in the far future
they no doubt will be somewhat different from us. But the theory of evolution
came to stay with us, and is not likely to be altered in the future. It’s
possible that some new ideas complement it, like spontaneous organization and
complexity. But the mechanic process of evolution wouldn’t need changes in this
case. And that would have to be treated in another essay…