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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.

Why have you chosen to work with choanoflagellates?

Choanoflagellates may hold clues to animal evolution.

King: Choanoflagellates are a window on early animal evolution. Both cell biological and molecular evidence indicate that choanoflagellates are the closest living relatives of multicellular animals.

Figure 1.

A choanoflagellate typically has a collar of tentacles and a single flagellum.
Image courtesy of the King Lab, University of California-Berkeley.

Choanoflagellates are a unique group of single-celled and colony-forming eukaryotes. There are at least 150 species of choanoflagellates, living in almost all aquatic habitats. Choanoflagellates use flagella to swim and trap food, mostly bacteria, in the walls of their collar (see image).

They may shed light on the transition to multicellularity.

The relationship of choanoflagellates to animals and the fact that they are unicellular suggest that they might help us understand the prehistory of multicellular animals. Their biology is similar to the hypothesized state of the unicellular ancestor of animals, so we think they have preserved this ancestral data better than other organisms. Genes shared by choanoflagellates and animals were likely present in their common ancestor and may shed light on the transition to multicellularity. Our lab has already provided evidence for the expression in choanoflagellates of protein families required for animal cell signaling [how cells communicate] and adhesion [how cells stick].

Did multicellularity evolve once or many times?

Each multicellular lineage arose independently.

King: Scientists have observed that the cell biology of multicellularity is radically different in different groups of organisms. So it suggests that different multicellular organisms arose from unicellular organisms numerous times. Animals, fungi, plants, and other multicellular lineages evolved multicellularity separately, and each lineage has a different common ancestor. This means that the mechanism by which multicellularity developed in each lineage is evolutionarily different and unique. When we focus on animals, however, we see that multicellularity evolved in this lineage only once.

Choanoflagellate genomes are evolutionarily unique.
Genomics research also benefits humans, in this case, cancer research.

Will you attempt to reconstruct the genome of the ancestor of choanoflagellates?

King: I don’t know if it will be technically feasible to do so entirely, but it’s something I like to think about. It’s a wonderful challenge for a scientist. Reconstructing the genome of the ancestor of animals and choanoflagellates would allow us to test whether we understand important components of the process by which animals evolved. One major challenge right now is to assemble choanoflagellate genomes. It is very interesting to work with an organism that is so distant from other organisms whose genomes have already been sequenced. There are no markers about where to go and how to proceed.

Our research into genome comparisons promises new insights into the last common ancestor of choanoflagellates and metazoans as well as the early evolutionary history of animals. Our research is in fact a study in macroevolution—trying to understand how major changes happened over large spans of time.

Beyond that, there may be some direct benefit to humankind. There is some interest in our work by people involved in cancer research. Many of the proteins that we are finding in choanoflagellates are ones that contribute to cancer development in humans. Our work may shed light on the cellular functions of some of these proteins.

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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