Who would have thought that a portable genetic test meant to expose sushi
restaurants trading tilapia for tuna would also provide important
information about evolution, including how new species form?
Who would have thought to look through five million of these "DNA
barcodes," or pictures of genes, gathered from 100,000 different animal
species by hundreds of researchers from across the world and stored in the
GenBank database operated by the US government?
That would be David Thaler of the University of Basel in Switzerland and
Mark Stoeckle from The Rockefeller University in New York, who jointly
released discoveries this week that are certain to shake up, if not
completely overthrow, a number of widely held beliefs about how evolution
occurs.
For instance, it is basic biology to assume that over time, populations of
big, dispersed species—think ants, rats, and humans—will diversify
genetically.
But is it actually the case?
Lead researcher Stoeckle, who was quoted as saying, "The answer is no," in
the study's journal article, Human Evolution.
Genetic diversity "is about the same" for the planet's 7.6 billion humans,
500 million house sparrows, or 100,000 sandpipers, he told AFP.
The study's most stunning finding may be that nine out of ten species,
including humans, first appeared on Earth 100,000–200,000 years ago.
Thaler told AFP, "This decision is quite unexpected, and I battled against
it as much as I could.
That response is understandable: How can one account for the fact that 90
percent of animal life is around the same age genetically?
Was there a terrible occurrence 200,000 years ago that almost destroyed
everything?
cheaper and easier
One has to understand DNA barcoding in order to comprehend the solution.
There are two types of DNA in animals.
Most animals get nuclear DNA, which is the type humans are most familiar
with. Nuclear DNA is handed down from male and female parents and includes
the genetic code for each person.
Four different types of molecules are organized in pairs to form the
genome, which is composed of DNA. There are three billion of these pairs in
humans, which are organized into 20,000 genes.
However, all animals also contain DNA in their mitochondria, which are the
tiny parts of every cell that transform food into usable energy.
37 genes are found in mitochondria, and one of them, COI, is employed for
DNA barcoding.
All animals have the same mitochondrial DNA, making it possible to compare
traits across species in contrast to the genes found in nuclear DNA, which
can vary significantly between species.
Additionally, it is far easier and less expensive to isolate mitochondrial
DNA.
By analyzing the COI gene, Canadian molecular researcher Paul Hebert—who is
credited with creating the phrase "DNA barcode"—discovered a method for
classifying species in the early 2000s.
The mitochondrial sequence has proven ideal for this all-animal strategy
because it perfectly balances two opposing characteristics, according to
Thaler.
"Neutral" genetic changes
The COI gene sequence is identical in all species, which makes it simple to
distinguish and contrast them.
However, these mitochondrial fragments are distinct enough to allow for the
differentiation of each species.
According to Thaler, "it almost exactly matches species designations made
by specialist experts in each animal domain."
The researchers discovered a telltale clue indicating that nearly all
animals arose about the same period as humans after analyzing the barcodes
of 100,000 species.
What scientists saw was a lack of variety in so-called "neutral" mutations,
which are minute alterations in DNA that have no effect on an individual's
likelihood of surviving.
In other words, they had no bearing on the sexual and natural forces that
drove evolution.
Like tree rings, the degree to which these "neutral" mutations resemble one
another indicates how old a species is.
This takes us full round to our original query: why did the vast majority
of species that exist today arise at roughly the same time?
Darwin was baffled.
According to Jesse Ausubel, head of The Rockefeller University's Program
for the Human Environment, environmental trauma is one potential.
"Viruses, ice ages, successful new competitors, loss of prey—all these may
cause periods when the population of an animal drops sharply," he said in
response to the findings, as reported by AFP.
"During these times, it is simpler for a genetic innovation to spread
throughout the population and help a new species emerge."
But 65.5 million years ago, a probable asteroid hit that wiped out half of
all species on Earth including the land-dwelling dinosaurs was the last
truly global extinction event. This indicates that a population "bottleneck"
is, at best, a limited answer.
The most straightforward explanation is that life is always changing,
according to Stoeckle.
"It is more likely that the animals that were alive at that time—at all
points in evolution—arose relatively recently."
According to this theory, a species only survives for a finite period of
time before either evolving into something new or becoming extinct.
Yet another surprise result of the study is that there is little genetic
overlap between species, which results in highly distinct genetic
borders.
According to Thaler, "if individuals are stars, then species are galaxies."
In the immensity of the void sequence space, they are compact
clusters.
According to Darwin, the lack of "in-between" species also confused
him.
