Genetic studies indicate that modern life descended from a common ancestor. The last universal common ancestor likely existed more than three billion years ago. Over the course of evolution, it appears that three main lines of descent—drawn as branches on the tree—have sprung from the last universal common ancestor and that all modern species would naturally fit into three large taxonomic groupings, called domains, based on these lineages.
Here we describe some of the reasons biologists group modern organisms into three domains. We also focus on a process called lateral gene transfer, in which one organism receives genetic information from another, without being the offspring of that organism. The evidence for lateral gene transfer has revealed surprising and complex webs of genetic relationships among the three evolving lineages.
In the past few years, scientists have sequenced the entire genomes of many different species from the domains Bacteria, Archaea, and Eukarya. This plethora of data has allowed scientists to compare how thousands of genes differ or are similar among organisms from the three domains.
Many of the comparisons corroborate the model of evolution that biologists currently hold: that Archaea and Eukarya diverged from the Bacteria long ago. However, many surprises have also emerged from the vast amount of collected data. For example, Archaean species have genes that have been recently derived from Bacteria, and Eukarya also have a number of genes that are of relatively recent Bacterial origin. This DNA sequence data has provided the evidence that lateral gene transfer occurred repeatedly throughout evolution.
Lateral gene transfer is a well-documented event that occurs today. It is clear that DNA can be transferred between organisms by vectors, such as viruses. Prokaryotes can also transfer DNA on circular pieces of vector DNA, called plasmids. Additionally, prokaryotes are also capable of taking up naked DNA through a process called transformation.
For scientists, the lateral transfer of genes has turned three relatively clean lines of descent into a vast and complex network of relationships. Yet it is debatable whether lateral gene transfer has seriously complicated attempts to resolve the tree of prokaryotic life. While it complicates studies in some individual species, it need not present problems at higher levels. Nucleotide sequence comparisons involving entire genomes are revealing a stable core of crucial genes that are uncomplicated by lateral gene transfer. Gene trees based on this stable core more accurately reveal relationships of the organismal phylogeny.
Textbook Reference: Concept 19.1 Life Consists of Three Domains That Share a Common Ancestor