Setting Back the Clock on a Simple Ancestor

    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 Science.
    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."