Mention the word evolution and the image of chimps morphing into humans might come to mind. This is just one small step, however. The process of evolution began millions of years ago. All forms of life known today, including plants, fungi and animals, arose originally from a sea of microscopic organisms.
In their Dec. 18 article in the Proceedings of the National Academy of the Sciences, Nicole King and Sean Carroll report evidence that the ancient ancestor of all animals is a group of protozoa known as choanoflagellates, a single-celled microbe that is dashing around your pond water with whiplike appendages.
It may be difficult to imagine that all the complexity of animal life could have evolved from a simple, one-celled organism. But choanoflagellates are not that simple.
\If you take a drop of pond water, you'll see lots of microorganisms leading complicated lives,"" King said. ""They divide, they change in response to the environment, they may eat a different kind of food depending on what's available, or they may build a protective coat. Microorganisms do complicated things, just not on the same order of magnitude as animals.""
This potential is hidden in the genes. New genes are not created as each new function of an organism evolves. Rather, over time, organisms reuse genes. By adapting the pre-existing genes to allow them to perform new and different functions, the organism becomes more complicated.
This process does not only happen on the level of genes, but also on a larger scale.
""Penguins took wings and used them for fins. Limbs evolved from fins,"" said David Baum, UW-Madison associate professor of botany. ""The same thing happens at the molecular level.""
Old genes are used for new functions. The same genetic building blocks are used in new combinations and with some modifications.
Watching the process of evolution at the level of genes allows scientists to track back evolutionary relationships between organisms. In order to provide evidence that animals arose from choanoflagellates, King and Carroll isolated four genes from the microbes. The sequences of these genes were compared to the analogous genes in various animals, fungi, plants and protists. These comparisons of the genetic code suggested a close relationship between choanoflagellates and animals.
Understanding the genetic basis of how a multicellular organism arose from a single-celled organism is especially intriguing. Lots of genes had to be modified into new genetic machinery to allow a single cell to give rise to an organism that is multicellular.
""It's good to find out which bits were added in what order,"" Baum said. ""Any complex characteristic, like multicellularity, or flight in birds ?? all of those traits couldn't have originated 'boom' all at once. There are a lot of different features that go into making something multicellular.""
As a result, many of those genetic modifications have to predate the origin of multicellularity.
""You expect the machinery of multicellularity to be there before multicellularity evolved,"" Baum said.
In King and Caroll's paper, they describe one piece of that machinery that was present before the evolution of multicellularity. In choanoflagellates, they found a signaling gene, a gene involved in relaying messages in a cell, that previously had only been identified in animals.
""Signal molecules arose before multicellularity,"" King said. Since signaling molecules are used for communication, it would be especially important in multicelled organisms because they would allow individual cells to talk to each other.
In choanoflagellates, signaling molecules may help them interpret their environment. Signal molecules help sense outside stimuli and signal the inside of the cell so it can be interpreted.
The role of these signaling molecules in choanoflagellates remains undetermined, however.
""We don't know how the signaling molecule is functioning. It may respond to extracellular signals, or it may have another role,"" said King.
Although the discovery of this signaling molecule in protozoa is new, King and Carroll's approach of looking to the genes to uncover the relationships between organisms is nothing new. Scientists used to depend solely on visible characteristics, like skeletal or cellular features, to infer evolutionary relationships between organisms. In modern studies of relationships between organisms, scientists combine the older approach of studying organisms' visible traits with comparisons of their genes.
Today, almost all attempts to reconstruct how organisms are related to each other use genetic data.
""The vast majority use this approach'it's much quicker, it's much more reliable, and you end up with much more convincing results at the end,"" Baum said.
According to Baum, there are two main advantages for looking to the genes when constructing evolutionary relationships, along with studying visible characteristics. One is the amazing amount of data that can be gathered from looking at the DNA. For example, while looking at a single-celled organism under the microscope, one notices there are just a few traits to distinguish it. Contrarily, while looking at the genes, one would see almost an infinite number of traits. Secondly, scientists can use more rigorous statistical approaches when comparing genes.
Looking at how the genetic code is modified as organisms evolve will help show us how humankind, and all the diversity of life seen today, could have arisen from a soup of tiny microbes.
