Common descent (evolutionary biology)

Common descent is a theory of evolutionary biology suggesting that all life on Earth shares a common genetic heritage. Common descent refers to the large-scale evolution of species over many generations, as opposed to small-scale evolution occurring from one generation to the next. English biologist Charles Darwin popularized the theory of common descent in his 1859 book On the Origin of Species. Darwin believed that all living organisms are descendants of a single species from the past. This single species is often referred to as the last universal common ancestor, or LUCA.

The existence of LUCA has been widely supported within the modern biological community since the mid-twentieth century, when scientists discovered the universality of DNA, or genetic makeup. Some scientists discount the existence of LUCA, however, relying on discoveries in cellular behavior to support a hypothesis suggesting multiple common ancestors.

All research supporting common descent remains circumstantial, however, due to the speculative nature of evolution.

Theory Development

The theory of evolution ties all life on Earth together through patterns of biological relationships between species. Common descent evolution proposes that all terrestrial organisms have a common ancestor, and is evident in the processes of artificial and natural selection.

Many modern plants and animals are the product of artificial selection, or selective breeding by humans. The products of artificial selection all share a common ancestor. For example, all dogs are descendants of the South Asian wolf. Dogs were later domesticated and selectively bred for various purposes.

Natural selection is the untainted evolutionary process by which animals and plants develop traits over time. An organism's environment determines natural selection. If a trait is favorable to the survival of a plant or animal, subsequent generations inherit it. Darwin observed natural selection on a trip to the Galapagos Islands. He noted that the beaks of finches differed throughout the island. He concluded that the beaks evolved out of necessity so that some birds had beaks that could crack through nuts and others developed beaks that could pick up insects. Despite these different traits, the birds all hailed from a common ancestor.

Evidence for Common Descent

Most organisms share an identical genetic code in their DNA. Scientists believe this code evolved early in evolutionary history and was inherited by all succeeding organisms. The universality of DNA gives strong support to the common descent theory, and other molecular similarities support it as well. Genome sequencing, a technique that allows scientists to understand the genetic information encoded in DNA, makes it possible to compare the genomes of different organisms. A whole genome comparison of humans to chimpanzees revealed a mere 1.23 percent sequence difference between the two. Similar examples appear in many other species. Structural similarities among organisms also lend strength to common descent theory. Some living things share comparable physical features. For example, all mammals possess the same basic bone structure of their forelimb (arm, wing, fin, etc.) regardless of its function (walking, flying, swimming, etc.).

Many scientists believe the presence of vestigial organs in organisms is also proof of common descent. Because these organs serve no function in the organisms in which they reside, their presence suggests that they were useful in an organism's evolutionary past but lost their value over time. Though organs are vestigial in some species, they play a crucial role in others, suggesting a link between these organisms.

The study of pseudogenes also supports the theory. Pseudogenes are genes no longer expressed in an organism. Scientists have found older sequences of the same pseudogenes in primates, leading to the conclusion that the sequences originated early in the evolutionary history of primates.

Scientists have also discovered chromosomes with similar blocks of genes present on different chromosomes in various organisms. For example, scientists found an identical block of genes on different chromosomes in mice and humans. Though the genes were found on different chromosomes, the order of the genes was the same, despite there being no reason for this.

Singular vs. Multiple Ancestors

Most common descent theorists advocate the single progenitor concept, which states that all life came from one universal common ancestor. Other theories hold that life evolved from multiple common ancestors. Though neither theory is scientifically proven, compelling evidence has been found for both.

Many biologists argue that the evidence points to a singular, universal common ancestor from which all modern life derives. Scientists believe this LUCA was a water-dwelling, single-celled organism that lived somewhere between 3.5 billion and 3.8 billion years ago and served as the tree from which all life grew. In 2010, biochemist Douglas Theobold came up with a method to statistically analyze amino acid sequences in several proteins transferred early in evolution. These proteins are present in all three domains of life (eukaryotes, bacteria, and archaea). His data found that the modern protein sequences more likely resulted from a singular common ancestor than from multiple ancestors.

Some biologists prefer a theory involving a common ancestral community of primitive cells. This concept outlines a lineage scenario in which multiple primitive cells evolved together, eventually breaking up into bacteria, archaea, and eukaryotes. Advanced understanding of cellular behavior gives credit to this theory. Microbiologists have learned that gene transfer in bacteria and viruses was much more complex than originally thought. Rather than simple vertical gene transfer from species to species, these microbes were capable of transferring genes horizontally and sideways. This complicated the idea of a tree of life, leading some scientists to work out a tree with multiple roots rather than a single root.

Bibliography

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