Setting Back the Clock on a Simple
When Charles Darwin came up with the theory that explained
biological evolution over a century ago, he was faced with a problem of
geological proportions. While he predicted that evolution was a slow
and gradual process that occurred over vast expanses of time, the fossil
record showed that virtually all the major body plans of the animals we
know today arrived at life's party simultaneously, in a narrow window of
time over 500 million years ago. Darwin said there must have been
a common ancestor to the cornucopia of creatures that originated in the
big bang of animal evolution, but after decades of searching, geologists
have yet to find hard evidence for true animal ancestors in older rocks.
Now, molecular biologists are coming to Darwin's
rescue. By reading time off molecular ‘clocks’ ticking away in the
cells of animals since their primordial birth, researchers are finding
that the earliest animal groups split off from a common ancestor as much
as half a billion years earlier than the fossil record leads us to believe.
The molecular evidence collected by researchers over the past twenty years
supports Darwin’s vision by suggesting that the tree of animal life had
much deeper roots.
Jeffrey Levinton, an evolutionary biologist at the
State University of New York and U.S. authority on the evolution of animals
said, "these clocks allow us to time the points in the distant past when
the most important innovations of animal life arose."
A molecular clock is a gene or protein common
in the cells of all living things, from bacteria to plants to animals.
According to some researchers, these molecules change, or mutate with clock-like
regularity. Over geological time mutations begin to build up within
the molecular clocks of different animal groups, or phyla, once they split
off from a common ancestor. Scientists calibrate the speed of these
genetic clocks by counting up the number of mutations that have accumulated
over a known interval of time, in specific groups of modern fish; skeletons
of the earliest fish are well preserved in rocks that have already been
dated using geological clocks. Thus by knowing the speed of the molecular
clock, researchers need only count up the number of genetic mutations in
a wide range of animal groups to project back to the time, before there
were fossils, when each phyla evolved. Animals grouped within a phyla
either look alike or have similar body plans. For example, on the
family tree of animals, fish and frogs and humans are all members of the
same phyla because they have spinal cords and bones.
A common animal ancestor has never been found in
the fossil record and probably never will. Paleontologist believe
that the earliest forms had no shells or other hard parts that could be
easily preserved. In fact, few animals living today would leave behind
recognizable fossils in rocks. The precursors of animals with shells
that appeared explosively at the base of the Cambrian Period may have been
microscopic, larval forms, said Gregory Wray, an evolutionary biologist
also at the State University of New York. They were likely "eency
weency things living between sand grains," he added.
The only traces of Darwin's ancestral animals may
be the tiny tracks and burrows left behind when worm-like creatures crawled
through muds on the sea floor in search of food. The soft-bodied
animals that made these markings, known as trace fossils, probably had
a head and a primitive circulatory and nervous system, as well as a rudimentary
gut, said Guy Narbonne, a paleontologist at Queens University in Canada.
But, like the fossil shells that appear suddenly in the Cambrian explosion
of animals, traces don't have a long history leading back to the time of
a common ancestor. "Even the trace fossils appear late in the game,"
said Narbonne. Further back in time the only living things paleontologists
have only been able to positively identify are seaweeds and single-celled
algae and bacteria.
The molecular search for the animal ancestors that
would prove Darwin's theory right began in 1982 when Bruce Runnegar, a
paleontologist at the University of California at Los Angeles, compared
the genetic code in the hemoglobin molecule from the blood of a selected
number of modern phyla. Relating the differences between the number
of hemoglobin mutations in different animal groups to time, Runnegar calculated
that the major body plans emerged from a simple ancestor some 900 to 1000
million years ago, almost twice the age suggested by the fossil record.
Today, the storehouse of genetic information on
animals has grown to the point that Gregory Wray and Jeffery Levinton with
their colleague Leo Shapiro used the genetic sequences in seven very different
molecules within 16 different phyla to push the primordial birth of animals
even further back in time, between 1000 and 1200 million years ago.
According to the authors, later modification of these basic body plans,
such as the evolution of skeletons and circulatory systems probably occurred
hundreds of millions of years later, nearer to the Cambrian explosion of
animals. These new results were published last October in the journal
While molecular clocks are the only tools researchers
have to date the earliest origin of animals, some argue that the clocks
don't keep very accurate time. "Relying on the molecular clock is
alot like believing in the Easter bunny," said Douglas Erwin, a paleontologist
at the Smithsonian Institution who studies the early fossil record of animals.
Erwin is skeptical of the very old ages indicated by the molecular clocks.
"I am very suspicious of claims that there is a long history of protracted
animal evolution before the Cambrian," said Erwin. Some researchers
suggest that the speed of genetic mutations may accelerate at times when
animals are evolving rapidly into different shapes and body plans.
Wray admits that the clocks "are neither extraordinarily accurate nor extraordinarily
precise," but while "the uncertainty makes it possible that the dates are
a bit too old, they still suggest that animal phyla evolved long before
the Cambrian explosion."
If the molecular sleuths are correct, it took soft-bodied
animals a long time to grow large and learn to protect themselves by building
shells around themselves. Still, the fact that animal phyla may have
evolved over half a billion years earlier does not diminish the importance
of the big bang of animal evolution, "it only means that the Cambrian explosion
had a longer fuse," said Andrew Knoll, a paleontologist at Harvard University.
Knoll and his colleagues believe that major climatic and environmental
changes at the surface of the Earth may have caused animals to grow in
size and start forming shells. These researchers have shown that
an origin of animals between 600 and 900 million years ago is likely, based
on new chemical evidence that suggests the amount of oxygen in the atmosphere
increased dramatically at this time.
By collecting and analyzing rocks formed in the
world's ancient oceans -- where Darwin's primordial animals evolved --
researchers have found chemical clues that suggests there were major blooms
of algae and bacteria in these seas. These algal blooms may have
caused oxygen to build up to near modern-day levels, from an earlier period
when there was very little oxygen around. The researchers theorize
that an increase in oxygen may have pushed biological evolution along by
allowing animals to get larger and become more active.
If Darwin were alive today he probably would have
carried around one of these molecular clocks in his pocket to prove to
his colleagues that there was a protracted history of animals before the
Cambrian, said Levinton. "But by now," he added, "Darwin would have
also realized that there is no theory that tells you how fast biological
evolution should go."