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Animals: Tracing Their Heritage

Nicole King


The earliest animals are absent from the fossil record, so scientists need to determine

  • the nature of the first animals by studying the basic characteristics shared among all animals
  • the origin of multicellularity, which is a pivotal event in animal evolution
  • how the closest living relative, the single-celled choanoflagellates, relates to multicellular animals

January 2007

Do animals have a common origin?

All animals have a common ancestor.

Purple-striped jellyfish (Chrysaora Colorata) at Montery Bay Aquarium, California. All animals can be traced to a common ancestor. Photo: Sanjay Acharya

King: Yes. All animals, from sponges to jellyfish to vertebrates [animals with a backbone], can be traced to a common ancestor. So far, molecular and fossil evidence indicate that animals evolved at least 600 million years ago. The fossil record does not reveal what the first animals looked like or how they lived. Therefore, my lab and other research groups around the world are investigating the nature of the first animals by studying diverse living organisms.

Most organisms on Earth have only one cell.

You study multicellularity. Is there a connection to animal origins?

King: Eukaryotes [organisms with membrane-bound nuclei] range from those with a single cell, such as the amoeba, to complex multicellular animals, including humans. The vast majority of life on Earth has been dominated by unicellular life. At some point in the lineage leading to animals, multicellularity evolved. Multicellular organisms are those that have many cells. Their cells depend on each other, functioning in concert to sustain the life of the organism. So, the common ancestor of animals was a single cell.

A single-celled organism gave rise to multicellular organisms.

It was that event—the origin of multicellularity— that was seminal to the evolutionary history of animals. We have yet to discover what this unicellular ancestor of multicellular animals was, but we have gathered clues about its genetic complexity. We don’t have a fossil record regarding the rise of multicellularity, but we can deduce the shared characteristics, using molecular and other data, among animals that are extinct and their living relatives.

A phylogenetic tree details the relationships among organisms.
Databases help us construct phylogenetic trees.

How does a phylogenetic tree allow you to make these connections?

King: A phylogenetic tree, or tree of life, is a diagram of the relationships among organisms. It is a hypothesis, always evolving as more data is added to it. Phylogeneticists take sequences of genes or other regions of genomes from diverse organisms and align them with each other to identify positions in the sequences that suggest shared ancestry. Those that have changed in concert with each other may suggest a common ancestor within that group to the exclusion of other groups.

This process used to be done by hand, but now computers have vastly accelerated the process. We now have publicly accessible databases of phylogenetic information that allow us to view and analyze gene sequences of diverse organisms.

Nicole King, Ph.D., is an assistant professor in the Department of Molecular and Cell Biology, University of California-Berkeley. King’s lab studies the origin and evolution of animals by trying to determine the minimal genomic complexity of the common ancestor of animals, elucidating the ancestral functions of genes required for multicellular development, characterizing choanoflagellate cell and developmental biology, and testing the hypothesis that the emergence of multicellular animals stemmed, in part, from the evolution of new modes of gene regulation. King was interviewed at the evolution symposium presented at the 2006 National Association of Biology Teachers annual conference, co-sponsored by the American Institute of Biological Sciences, the National Evolutionary Synthesis Center, and the Biological Sciences Curriculum Study publishing house.

Animals: Tracing Their Heritage


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