The Shape of Life

For the first time ever, scientists believe they have gathered substantial evidence that points to a single animal group of creatures that gave rise to all animals, including humans. Researchers such as Cristina Diaz and Mitch Sogin think that the most likely candidate for this "Animal Eve" is a group of creatures that still exist: the sponges. Sponges, members of the phylum Porifera, are considered the oldest living animal phylum. The name Porifera means "pore bearer" in Latin. Sponges are the only animals that if broken down to the level of their cells can miraculously reassemble and resurrect themselves. These seemingly inanimate creatures are also fantastic pumps, filtering tons of water to harvest just a few ounces of microscopic food. How do we know sponges were our ancestors? It turns out that all organisms in their genes carry clues to their evolutionary history -- a unique set of acquired genetic changes passed on through countless generations. This fact allowed Mitch Sogin to compare and contrast specific sets of genetic differences between sponges, flies, fish, frogs, humans and other organisms. He discovered that sponges, indeed, were the start of the animal kingdom and laid the foundation for all animals to follow.

When we think of animals, we think of movement. Surprisingly, the diverse and graceful ballet of animal movement may have started with cnidarians (pronounced "ny-DAIR-ee-ans), a group that includes corals, sea anemones, sea pens and jellyfish. All of these animals, with few exceptions, have nerves and muscles. Because cnidarians are the simplest animals to possess this complexity, their direct ancestors were very likely the first animals to bundle the power of nerves and muscles together, enabling them to move and exhibit discernible behavior. Cnidarians are also the first animals with an actual body of definite form and shape. Most feature tentacles with stinging cells used to capture prey. The cnidarian's sting comes from tiny, often toxic harpoons called nematocysts. Triggered by touch or by certain chemicals, nematocysts fire out of the stinging cells at lightning speed.

The best way to find food is to go out and hunt for it. But to hunt, you need to be able to move forward. And to move forward, you usually need a head with paired sense organs to know where you are going attached to a symmetrical body to get you there. Scientists believe that a flatworm-like animal was the first creature to develop a head, brain, paired senses and a tail, the first to move forward and thus the first to hunt for food and mates. This breakthrough in bilateral design was enormous. It granted these creatures huge advantages over animals that could only sit and wait for food to float to them, e.g. sponges and polyps, or simply pulse aimlessly through the water in search of a meal, e.g. jellyfish. Modern representatives of this pioneering ancient design are the flatworms -- a group of animals in the phylum Platyhelminthes (pronounced "pla-tee-HEL-minthes"). This unsung group includes such animals as freshwater planaria, psychedelic marine polyclads and parasitic tapeworms and flukes. Platyhelminthes may not be famous, but they certainly aren't rare. About 20,000 kinds of flatworms are alive and well today, in fresh and salt-water environments and other nice damp niches, like the insides of other animals.

For hundreds of millions of years, animal life resided only in the oceans. And then about 400 million years ago, fossil tracks suggest that an animal called a eurypterid left the water to walk on land. Maybe it was fleeing enemies, maybe it was searching for an easy meal, or maybe it was seeking a safe place to lay its eggs. Eurypterids were members of a larger group that would invade the land many times over -- a group known as the arthropods. Defined by segmented bodies, jointed legs and hard exoskeletons, arthropods comprise over 80% of the animal species living today. Many are familiar, such as crabs, lobsters and of course, insects. Paleontologists have recently discovered that arthropods invaded land not once but many times. Fossil evidence shows that different groups including insects, millipedes and centipedes, spiders and scorpions -- all came ashore on their own at different times. With appendages adapted for walking, breathing, pinching, sensing and ultimately flying, arthropods have achieved amazing success on land. Indeed, the development of flight was the tour de force of the arthropod insect branch and perhaps the single most important adaptation that allowed them to eventually dominate every habitable ecosystem on Earth.

Behind the beautiful shapes and colors of seashells is the story of how a group of animals called molluscs evolved in order to survive. The wide variety of molluscs includes clams, oysters, snails, mussels, squid, and octopus. The word mollusc comes from Latin meaning "soft," a good description of the group's fleshy bodies. Of course, in an ocean filled with predators, a soft body is easily eaten. The early molluscs that happened to develop hard shells not only managed to survive but also succeeded in launching an ever-escalating 500 million year old battle between themselves and their predators. Molluscs have survived throughout the millennia by having an immensely adaptable body plan. One example of this is demonstrated by today's Nautilus. The ancestors of Nautilus evolved buoyant shells, a trait that allowed them to launch off the seafloor and become swimming predators known as cephalopods. As each generation struggled against increasingly clever predators like vertebrates, the cephalopods accumulated more and more sophisticated innovations through evolution. One adaptation lay in speed. In creatures such as squid, the shell became smaller, moved inside the body and all but vanished. Another adaptation lay in brainpower. Octopuses and cuttlefish think, learn and react to their environments in ways surpassed only by vertebrates. If you have a soft body without a shell in the middle of the ocean, being clever is certainly one worthwhile strategy for survival.

It's easy to believe that animals like us, creatures with heads, eyes, and brains, are the crowning achievement of evolution. But are we really? Echinoderms like sea stars, sea urchins, and sea cucumbers have no head, eyes, or centralized brains, yet they have proven themselves worthy competitors. In fact, they can live in places inaccessible to many other animals. Echinoderms evolved like no other animals on Earth. Most animals evolved bilateral bodies equipped with a head, central nervous system and brain. But echinoderms opted for a different path. They adopted a five-part symmetry with no head to lead the way. Their bodies seem little more than a skeleton made of tiny little plates and water. They don't use large muscles working on large body parts like other animals. Instead they move on hundreds of tiny, water-filled tube feet operated by a hydraulic system that doesn't produce high-speed movement. Most move so slowly that, by our standards, they appear to be nearly stationary. Scientists and now filmmakers have managed to bring these animals to vibrant life -- all through the use of time-lapse photography. When sped up, these animals spring into action competing for dominance, fighting for food, and hunting down prey just like lions on the Serengeti.

In the 4.5 billion year history of Earth, a mere 10 million years seems rather insignificant, the equivalent of two months in the life of a 75 year-old man. Yet, during a 10 to 20 million year stretch of time, beginning about 540 million years ago, life evolved at an explosive rate. Scientists call the period the "Cambrian Explosion." Paleontologists believe that before this explosion began, the only animals on Earth were sponges, cnidarians and ancestral bilateral worms. Yet by the end of the Cambrian explosion, all of the eight major animals body plans in existence today, along with 27 minor ones, had emerged. And no new body plans have developed since. A group of animals called annelid worms developed during the Cambrian Explosion. Today, about 15,000 species of annelids exist including earthworms, marine bristle worms, and leeches. Scientists believe that burrowing worms play a vital role in maintaining life on Earth by recycling plant and animal remains into carbon dioxide gas. This gas helps modify the climate of the biosphere. Before active burrowers appeared, organic remains became buried in sediments and depleted the atmosphere of carbon dioxide. Actively feeding worms however, recycle buried organic material in a timely basis releasing carbon dioxide back into the atmosphere. With sufficient carbon dioxide in the air, land plants can thrive and the oceans remain free of ice across much of the planet.

As we go through our lives -- driving cars, exploring the Internet, studying the world around us -- it is hard to imagine that we're related to Earth's other animals. It's even a stretch to see what connects us with the rest of the chordates, a group of about 50,000 species including the vertebrates like fish, amphibians, reptiles and birds, mammals and ourselves. But indeed, all chordates, from the worm-like amphioxus to Homo sapiens have three common features. Each of us has a single hollow nerve bundle running up our backs that blossoms into a brain in the head of most chordates. Each also has a stiff rod, called a notochord, that contains fluid-filled cells sheathed in fibrous tissue. (In humans, as in most vertebrates, the notochord becomes part of the structure of the discs between the vertebrae of our spines.) The third trait shared by all chordates is the presence, at some stage of life, of gill slits in the throat. (Human gill slits close up while we're still embryos.) Chordates, and particularly back-boned vertebrates, have achieved incredible success on Earth. Scientists attribute much of that success to evolutionary modifications in their genetic make-up. Vertebrates have many more genes than our invertebrate relatives. The additional set of genes have allowed us to develop complex body parts -- the backbones, skulls, jaws, teeth and sophisticated brains -- that have enabled vertebrates to dominate land, sea and air. Before we as humans gloat over our success, we must remember we are not the sole survivors of this great journey of evolution. We are part of an epic story much larger than ourselves.