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Life arose just once on Earth; all other species evolved from this common ancestor. Each branch on the tree of life represents a new species emerging and passing its features on to its descendents. If every species on Earth today were included, this tree would have many millions of branches.
Life was born in the sea, and every major evolutionary branch began there. From the first microbial organism to the first multicellular animal and every major plant and animal phylum, or primary group, the evolution of life on Earth took place in the sea. This tree introduces the major groups alive today; many others appeared and died out long ago.
An evolutionary tree, or cladogram, shows which species and groups of species are most closely related. Species that branch from a common point, or node, share similar features because they descended from a common ancestor. Smaller groups connect into larger groups at nodes lower on the tree. This series of nesting groups ultimately leads back to the common ancestor of them all.
Every node represents a point in evolution when a new species arose and some new feature appeared. Each new species passed on its characteristics to its descendents. As more new species evolved, more branches split off the tree.
The two evolutionary trees in this hall fit together. The entire vertebrate tree stems from a single branch in the larger tree—the Chordata, animals with a nerve chord in their back. The vertebrate tree evolved over the past 500 million years; the broader marine tree covers much more time, about 1.5 billion years. The first microbial life, including bacteria and archaea, arose about 2 billion years before then.
Just a few important events in the history of evolution are shown on this tree.
Metozoa: Multicellular animals with a system of communication between cells using chemicals still found in the human nervous system.
Bilateria: First appearance of animals with bilateral (two-sided) symmetry. Previously, animals had radial (circular) symmetry.
Ecdysozoa: A group of animals that molt, or shed outer layers. Molting lets animals with external skeletons grow and change shape.
Trochozoa: A diverse group of mollusks and worms that pass through a larval stage with two bands of hairlike cilia for swimming.
Chordata: Animals with a nerve chord and a central rod in their back, from which the vertebrate backbone and spinal chord evolved.
The invertebrate models on display here are just a small sample of the millions of species living today. Lines branching from a single point on the evolutionary tree represent lineages that share features evolved from a common ancestor.
Looking for features shared by different groups involves detailed study of a species' structure, behavior and genes. Genetic data have revealed that while some invertebrate groups look alike, they are not closely related. Meanwhile organisms that don't resemble each other have turned out to be close relatives. As scientists continue to unveil these types of new relationships, we gain more and more knowledge about the origins of life on Earth.
The following sections introduce two major groups on the invertebrate tree: algae, sponges and jellies, and mollusks, echinoderms and arthropods.
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