[Paleopsych] NYT: Plain, Simple, Primitive? Not the Jellyfish

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Plain, Simple, Primitive? Not the Jellyfish
http://www.nytimes.com/2005/06/21/science/21jell.html

    By [3]CARL ZIMMER

    Jellyfish have traditionally been considered simple and primitive.
    When you gaze at one in an aquarium tank, it is not hard to see why.

    Like its relatives the sea anemone and coral, the jellyfish looks like
    a no-frills animal. It has no head, no back or front, no left or right
    sides, no legs or fins. It has no heart. Its gut is a blind pouch
    rather than a tube, so its mouth must serve as its anus. Instead of a
    brain, it has a diffuse net of nerves.

    A fish or a shrimp may move quickly in a determined swim; a jellyfish
    pulses lazily along.

    But new research has made scientists realize that they have
    underestimated the jellyfish and its relatives - known collectively as
    cnidarians (pronounced nih-DEHR-ee-uns). Beneath their seemingly
    simple exterior lies a remarkably sophisticated collection of genes,
    including many that give rise to humans' complex anatomy.

    These discoveries have inspired new theories about how animals evolved
    600 million years ago. The findings have also attracted scientists to
    cnidarians as a model to understand the human body.

    "The big surprise is that cnidarians are much more complex genetically
    than anyone would have guessed," said Dr. Kevin J. Peterson, a
    biologist at Dartmouth. "This data have made a lot of people step back
    and realize that a lot of what they had thought about cnidarians was
    all wrong."

    Renaissance scholars considered them plants. Eighteenth-century
    naturalists grudgingly granted them admittance into the animal
    kingdom, but only just. They classified cnidarians as "zoophytes,"
    somewhere between animal and plant.

    It was not until the 19th century that naturalists began to understand
    how cnidarians developed from fertilized eggs, their body parts
    growing from two primordial layers of tissue, the endoderm and
    ectoderm.

    Other animals, including humans and insects, have a third layer of
    embryonic tissue, the mesoderm, wedged between the ectoderm and the
    endoderm. It gives rise to muscles, the heart and other organs not
    found in cnidarians.

    Cnidarians also have a simpler overall body plan. Fish, fruit flies
    and earthworms all have heads and tails, backs and fronts, and left
    and right sides. Scientists refer to animals, including humans, with
    this two-sided symmetry as bilaterians. In contrast, cnidarians seem
    to lack such symmetry completely. A jellyfish, for example, has the
    symmetry of a bicycle wheel, radiating from a central axis.

    Evolutionary biologists came to view cnidarians as relics from the
    early days of animal evolution. The first animals were probably
    spongelike, little more than clumps of cooperative cells. Cnidarians
    seemed to represent the next stage, having acquired traits like simple
    tissues and nerves.

    The fossil records of animals seemed to back up that hypothesis. Many
    of the earliest animal fossils resembled jellyfish or other
    cnidarians. The oldest known bilaterian fossils were younger,
    appearing in the so-called Cambrian explosion, 540 million years ago.

    Some researchers suggested that the bilaterian body plan helped set
    off the Cambrian explosion. Unlike their ancestors, bilaterians had
    heads, allowing them to sense their surroundings and control a
    swimming, or crawling, body.

    Recent research undermines that theory. The oldest fossils that can be
    confidently called cnidarians are just 540 million years old. And Dr.
    Peterson and his colleagues have made new estimates of the age of
    cnidarians by studying their DNA.

    The DNA mutates at a roughly regular rate over millions of years, a
    so-called molecular clock. Dr. Peterson estimates that the common
    ancestor of living cnidarians lived 543 million years ago. In other
    words, cnidarians did not appear tens of millions of years before
    bilaterians.

    Genetic studies have also challenged conventional theories about
    cnidarians. Beginning in the 1980's, scientists studying bilaterians
    uncovered a set of genes that laid out their body plan. Some of the
    genes established the head-to-tail axis, others distinguished the
    front from the back.

    Humans and insects may look very different, but they share almost
    identical versions of this genetic tool kit. And the findings
    suggested that the tool kit had already evolved in the common ancestor
    of bilaterians.

    Dr. Mark E. Martindale of the University of Hawaii and his colleagues
    decided to look for the genes that build jellyfish and other
    cnidarians. It took a long time to begin generating results. The team
    had to find a species that could not only survive life in the
    laboratory but could produce enough embryos for research.

    Dr. Martindale's group chose the starlet sea anemone, a species found
    along the New England coast. Figuring out how to culture the anemone
    and investigate its genes demanded great patience. "It's taken 9 or 10
    years," Dr. Martindale said, "but it's turned into a gold mine."

    Much to their surprise, the scientists found that some genes switched
    on in embryos were nearly identical to the genes that determined the
    head-to-tail axis of bilaterians, including humans. More surprisingly,
    the genes switched on in the same head-to-tail pattern as in
    bilaterians.

    Further studies showed that cnidarians used other genes from the
    bilaterian tool kit. The same genes that patterned the front and back
    of the bilaterian embryo, for example, were produced on opposite sides
    of the anemone embryo.

    The findings have these scientists wondering why cnidarians use such a
    complex set of body-building genes when their bodies end up looking so
    simple. They have concluded that cnidarians may be more complicated
    than they appear, particularly in their nervous systems.

    "At the molecular level, they have a lot of body regions that aren't
    recognizable," said Dr. John R. Finnerty, a biologist at Boston
    University who is collaborating with Dr. Martindale.

    Dr. Finnerty expects that the nervous system of cnidarians will turn
    out to be particularly complex. "The nervous system of a cnidarian is
    described as a nerve net, but that's a textbook simplification," he
    said.

    He predicts that research will show that this net is divided into
    specialized regions like the human brain.

    These discoveries have prompted Dr. Peterson to reconsider cnidarians'
    place in the history of life. "It's changed my thinking about early
    animal evolution," he said.

    He now theorizes that cnidarians were not the simple forerunners of
    the Cambrian explosion, but very much part of it, their evolution
    driven by the rise of animal food webs.

    In a paper to be published in the journal Paleobiology, Dr. Peterson
    and his colleagues propose that the common ancestor of bilaterians and
    cnidarians is a crawling worm. This ancient worm, which Dr. Peterson
    estimates lived 600 million years ago, represented a major advance in
    animal evolution. Instead of passively filtering tiny bits of food, it
    was able to graze on larger prey.

    "Once they're able to start grazing through those microbial mats,
    there's nothing to stop them," Dr. Peterson said.

    Some of these animals eventually began to eat one another. Animals
    that could defend themselves were more likely to survive. One way to
    avoid being eaten was to become bigger. Another way was to loft eggs
    into the water column rather than leave them to be eaten on the sea
    floor. Some animals even began to swim as adults in the open water.

    Once the water began to fill with animals, cnidarians took on their
    current form. The earliest cnidarians anchored themselves to the sea
    floor and grew upward, as sea anemones and corals do today. In the
    process, they abandoned the bilaterian body plan of their ancestors.

    It was also at that time that cnidarians evolved their distinctive
    weaponry: a cell containing a miniature harpoon called a nematocyte,
    for paralyzing prey with toxins.

    As new lineages of animals moved even higher into the water column,
    some cnidarians evolved to hunt them as well. Jellyfish are the
    product of this final stage of evolution, Dr. Peterson argues.

    These new insights into cnidarians have prompted major initiatives to
    understand them better. The starlet sea anemone genome is being
    sequenced by the Energy Department's Joint Genome Institute, and is
    expected to be complete this year.

    Scientists expect a number of surprises from the genome project. They
    have already discovered that a number of genes once thought to be
    unique to vertebrates have turned up in the genomes of cnidarians. It
    is now clear that these genes did not, in fact, arise in early
    vertebrates.

    They are much older, having evolved in the common ancestor of
    cnidarians and bilaterians 600 million years ago. Later, they
    disappeared in branches of bilaterians like insects and nematodes that
    have been the focus of extensive genome research.

    In some ways, cnidarians are a better model for human biology than
    fruit flies. As strange as it may seem, gazing at a jellyfish in an
    aquarium is a lot like looking in the mirror.