Redoing the Tree of Life Again
| Author: Stephen Caesar Subject: Biology Date: 10/21/2006 |
One of the best-known images in the study of biological origins is the Tree of Life, which portrays the theorized evolutionary journey of all life forms from a single-celled organism. According to this model, life evolved in a linear fashion from the first life form and branched out into the separate kingdoms, phyla, and so forth that we see today. However, as with so many other theories regarding the origins of life, this famous image has proved incorrect.
Ford Doolittle, professor of biochemistry and molecular biology at Dalhousie University in Nova Scotia and director of the Program in Evolutionary Biology of the Canadian Institute for Advanced Research, wrote in Scientific American:
“Charles Darwin contended more than a century ago that all modern species diverged from a more limited set of ancestral groups, which themselves evolved from still fewer progenitors and so on back to the beginning of life. In principle, then, the relationships among all living and extinct organisms could be represented as a single genealogical tree.”
Most contemporary researchers agree. Many would even argue that the general features of this tree are already known, all the way down to the root—a solitary cell, termed life’s last universal common ancestor, that lived roughly 3.5 to 3.8 billion years ago. The consensus view did not come easily but has been widely accepted for more than a decade. “Yet ill winds are blowing. To everyone’s surprise, discoveries made in the past few years have begun to cast serious doubt on some aspects of the tree, especially on the depiction of the relationships near the root” (Doolittle 2000: 90).
According to prevailing theory, the common ancestor of all living things spawned both the kingdom of bacteria and the kingdom of archaea. The archaea in turn gave rise to the eukaryotes (Ibid. 93). However, in Doolittle’s words, genetic studies have shown that “the pattern of evolution is not as linear and treelike as Darwin imagined it” (Ibid.).
At this point we must pause to explain terms. In the 1960’s, genetic studies by Carl Woese of the University of Illinois showed that living things could be classified into two groups: eukaryotes and prokaryotes. Eukaryotes, which include humans, animals, plants, fungi, and some single-celled organisms, are composed of cells that contain a true nucleus, which houses the cell’s chromosomes (Ibid. 91). Eukaryotic cells also possess a cytoskeleton, a complex system of internal membranes, and mitochondria (organelles that use oxygen to extract energy from nutrients). Prokaryotes, on the other hand, consist of smaller and simpler cells without a nucleus, and are usually enclosed by a membrane and a rigid outer wall. Most prokaryotes are bacteria (Ibid. 92).
By the late 1970’s, Woese’s ongoing studies led him to alter this model of the tree of life. He found that certain prokaryotes that he had thought were bacteria were genetically much different, in addition to displaying vastly different behavior from bacteria. Woese proposed that these strange one-celled creatures were actually a third branch to add to the prokaryotes and eukaryotes, the archaea, “as different from bacteria as bacteria are from eukaryotes,” in Doolittle’s words (Ibid.).
Evolutionists theorized that the archaea evolved in linear fashion into today’s eukaryotes. However, thanks to the discovery of “gene-swapping” among different species of bacteria, microbiologists now know that this straight, linear tree-pattern is incorrect. “Gene-swapping” occurs when the DNA of one species of bacterium is transferred to another, greatly altering the receiving species without any evolution being involved. In fact, gene-swapping is how some disease-causing bacteria pass on antibiotic resistance to other species of infectious bacteria (Ibid. 94).
This phenomenon has shown that the traditional model of the tree of life, with its pattern of linear evolution, must be replaced. Prof. Doolittle comments:
“What do the new findings say about the structure of the universal tree of life? One lesson is that the neat progression from archaea to eukaryote in the consensus tree is oversimplified or wrong….[W]e must now admit that any tree is at best a description of the evolutionary history of only part of an organism’s genome. The consensus tree is an overly simplified depiction” (Ibid.).
It is amazing to think that one of Darwin’s most prominent concepts, and one of evolution’s most widely recognizable images, has been disputed by these discoveries. This sea change in the search for origins shows how easy it is for widely held theories to be overthrown despite having been adhered to by so many scientists for so long.
Reference:
Doolittle, W. F. 2000. “Uprooting the Tree of Life.” Scientific American, vol. 282, no. 2.
Stephen Caesar holds his master’s degree in anthropology/archaeology from Harvard. He is a staff member at Associates for Biblical Research.