New research provides more evidence that ancestors of modern choanoflagellates may be the evolutionary bridge linking single-celled and multi-celled organisms. Choanoflagellares are single-celled eukaryotic cells classified as protozoans; tiny organisms that more closely resemble animals than plants. These tiny organisms are also very flexible in their living arrangements. Modern day choanoflagellates can live independently or may exist in colonies. These microbes are of special interest to humans, because the single-celled ancestors of animals are thought to have resembled modern choanoflagellates.
In form, choanoflagellates bear a resemblance to the individual cells of sponges (tiny organisms that live in colonies). Each choanoflagellate possesses a single whip-like projection called a flagellum, which aids in securing food, and, for free-living choanoflagellates, provides a means of locomotion.
* Unicellular and Multi-cellular Organisms *
Organisms that can exist both independently and in groupings of cells are of interest to scientists because, at some point in evolution, primitive single-celled microbes gave rise to multi-celled organisms.
All living things are made of one or more cells; each cell, the common denominator of structure and function. To function properly, a cell must be able to transmit signals and transport materials intracellularly (within the cell), somewhat like a little factory that works to carry out the job of living and reproducing.
Multi-cellular organisms require a much more sophisticated system for signaling and transporting materials. Cells working together must be able to communicate not only within the cell, but also between cells (intercellularly), analogous to a network of factories working together to produce something that none of them could accomplish on their own.
* Chanoflagellates: The Missing Link *
There are several reasons why the ancestors of choanoflagellates have long been thought to be the missing link between single-celled and multi-celled organisms. These little critters are similar to the individual cells of the colonies that comprise ocean sponges. Furthermore, although there are many microbes that possess flagella, the flagellum of a choanoflagellate actually pushes it along (much like the flagellum of a sperm), whereas most flagellates are instead pulled by the action of their flagella.
These characteristics suggest a relationship between choanoflagellates and more evolutionarily advanced organisms. Now, a paper recently published in the Proceedings of the National Academies of Science provides more evidence supporting the link.
* New Research Findings on Choanoflagellates *
The choanoflagellate genome has recently been sequenced, revealing genes that code for special proteins, similar to those found in the cells more complex organisms, including humans. In multi-cellular organisms, these proteins are used in intercellular (cell to cell) communication.
* Sources *
National Science Foundation (2008). ‘New Evidence That Ancient Choanoflagellates’ Form Evolutionary Link Between Single-celled And Multi-celled Organisms.’ ScienceDaily, 3.
King, Nichole (2005). ‘Choanoflagellates’. Current Biology. Vol. 15, Issue 4, 22.