Do animals have a common origin?
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.
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.
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.
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.