Common Ancestry

The theory of common ancestry

Darwin proposed that similarities among species could mean that they are related and could be traced back to a common ancestor that evolved into new species due to adapting to their specific environment.

For example, Darwin hypothesized that all of the different finch species on the Galapagos Islands came from one parent species that first colonized the islands millions of years ago. Darwin explained that, as populations of the ancestral species spread from one uninhabited island to the next, they adapted to different ecological niches and rapidly evolved into many descendants.

Darwin came up with this hypothesis from his observation that the finches had very similar traits and were only different in terms of beak shapes and feeding habits that allowed them to adapt to their specific environment.

Figure 1. A diagram showing how a parent species of finch rapidly formed several new species of finch with different beak shapes and feeding habits.

From this example, we can see that, throughout evolution, ancestral species branch out into new species. As we go back in geologic time, species can be traced back to a smaller and smaller group of common ancestors. By extension, the theory of common ancestry holds that all life forms descended from one "universal common ancestor". To quote Darwin:

The "universal common ancestor" is commonly referred to as LUCA (Last Universal Common Ancestor). LUCA is believed to have lived between 3.5 and 4.5 billion years ago. LUCA was not the first living organism but rather the earliest known common ancestor of all currently living species.

Evidence of Common Ancestry of Life

Similarities shared by organisms, and patterns in the fossil record, provide evidence of common ancestry. This section will discuss homology and fossils as evidence of common ancestry.

Similarity Resulting From Common Ancestry Is Known as Homology

Similar traits and features among various species can provide evidence of common ancestry. It is likely that traits and features shared by a group of organisms were inherited from a common ancestor.

Similar traits and features due to common ancestry are known as homology. By studying the homology of organisms, we can infer how they are related. The more similarities organisms share, the more closely related they likely are.

There are three types of homology: morphological, molecular, and developmental homology. Each of these will be briefly discussed in the following section.

Morphological homology

In morphological homology, similarities can be observed in the structure and form of the species. For example, mammals can be classified as monotremes, marsupials, and placentals based on similar features:

• Monotremes, like platypuses, are mammals that lay eggs.

• Like rodents, dogs, and whales, Placentals are mammals with a placenta, a temporary organ that connects the embryo to the mother's uterus.

• Like kangaroos, wombats, and koalas, marsupials use external pouches to raise their newborn offspring.

Organisms under each group, monotremes, placentals, and marsupials, are classified as such because they share similar structures and can be traced back to a common ancestor.

Figure 2. A picture showing different mammals that all have pouches. They are collectively called marsupials.

Molecular Homology

In molecular homology, similarities can be observed in the genes or DNA sequence of the species. These similarities could result in similar observable traits, but this is not always the case; there are instances where two or more species have major morphological differences but have nearly identical genes. For this reason, genetic information like DNA is important evidence of common ancestry.

For example, Hawaiian silversword plants across the islands of Hawaii look very different, but their genes are very similar.

Figure 3-4. Dubautia linearis (left) and Argyroxiphium sandwicense (right) are two species of Hawaiian silversword plants that look morphologically different but are genetically similar.

Additionally, all life forms share the same genetic material. From bacteria to humans, all life forms have DNA and its mechanism for replication and expression, suggesting that all species came from a very distant common ancestor.

Developmental Homology

In developmental homology, similarities can be observed in particular developmental stages of the organisms. For example, all vertebrate embryos (even humans!) have gill slits and tails that disappear by the time of birth. We can infer that all vertebrates can be linked to a common ancestor.

Figure 5. We can see a tail in this photo of a 5-week old human embryo.

Patterns in the Fossil Record Provide Evidence of Common Ancestry

Fossils are preserved remnants or traces of organisms from a past geologic age. They show how gradual changes in the features of pre-existing organisms led to the formation of new species over time. When we look at fossils farther back in geologic time, we can trace the origins of today's organisms. Through fossils, we can also link traits of organisms to the traits of their ancestors, even those that no longer exist today.

For example, we know that cetaceans (an order of marine mammals that includes whales, dolphins, and porpoises) evolved from terrestrial mammals like hippopotamuses, pigs, and cows because the fossil record shows that flukes and flippers of cetaceans were derived from their extinct ancestors’ pelvis and hind bones that gradually shrunk over time.

Figures 6-7. Fossils show that the hippopotamus (left) is the closest living relative of the whale (right).

By observing similarities among species and patterns in the fossil record, we can infer how species are related, where they originated, and how their features changed in evolution. Inferences about the common ancestry of different species can be visualized through phylogenetic trees.

Figure 8. Phylogenetic trees show the evolutionary history and common ancestry of different species.

Most Similarities in Morphology, Fossils, and Embryos Are Results of shared DNA - an Outcome of Common Ancestry

Similarities in morphology, fossils, and embryos of organisms all boil down to shared DNA or genetic information--the immediate outcome of common ancestry. Organisms' observable traits are determined by their genetic information and their interaction with the environment.

Morphological and developmental similarities among related organisms are expressions of shared DNA. Likewise, similarities in fossils--as remnants of organisms--can also be traced back to shared DNA.

How Does Common Ancestry Provide Evidence for Evolution?

Common ancestry is an important component of evolution because it shows that new species emerge from pre-existing species, meaning life forms change over time. Common ancestry also shows that one ancestral population could diversify into many descendant species with modifications that are more suited to their present environment.

Common ancestry illustrates the unity and diversity of life brought about by evolution by natural selection.