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Common Ancestry

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Biology

How are different life forms related? Here, we will discuss the definition of common ancestry and the lines of evidence that support common ancestry. In addition, we will look at how common ancestry provides evidence for evolution.

What Does Common Ancestry Mean?

Common ancestry (also referred to as common descent) means having descended from one ancestor. It results in new species forming from one ancestral population due to evolution.

Sharing a recent common ancestor means that two or more species are closely related. On the other hand, not having a recent common ancestor implies that two or more species are more distantly related.

We say "distantly related" because it is believed that all life forms can be traced back to one common ancestor. This idea is usually referred to as the common ancestry of life, and it is a central concept in Darwin’s book, On the Origin of the Species.

What is the theory of common ancestry?

The theory of common ancestry holds that all life forms descended from one "universal common ancestor".

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 (Fig. 1).

Common Ancestry Fig. 1: Adaptive radiation in Darwin's Finches | StudySmarter

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. Source: National Human Genome Research Institute's Talking Glossary, Public domain, via Wikimedia Commons.

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:

"I should infer from analogy that probably all the organic beings which have ever lived on this earth have descended from some one primordial form, into which life was first breathed."

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.

Is There 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 (Fig. 2).

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.

Common Ancestry Fig. 2: Marsupials are mammals that use external pouches to nurse their young | StudySmarterFigure 2. A picture showing different mammals that all have pouches. They are collectively called marsupials. Source: John Cummings, CC BY-SA 3.0, via Wikimedia Commons.

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 (Figs. 3 & 4).

Common Ancestry Fig. 3: Dubautia linearis | StudySmarter

Common Ancestry Fig. 4: Argyroxiphium sandwicense | StudySmarter

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. Source (left): Forest & Kim Starr, CC BY 3.0 US, via Wikimedia Commons. Source (right): The original uploader was KarlM at English Wikipedia., CC BY-SA 2.5, via Wikimedia Commons.

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.

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 (Fig. 5).

Common Ancestry Fig. 5: Human embryo | StudySmarterFigure 5. We can see a tail in this photo of a 5-week old human embryo. Source: Ed Uthman, MD (Flickr, Wikipedia), Public domain, via Wikimedia Commons.

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.

Common Ancestry Fig. 5: Hippo | StudySmarter

Common Ancestry Fig. 7: Whale | StudySmarter

Figures 6-7. Fossils show that the hippopotamus (left) is the closest living relative of the whale (right). Source (left): Diego Delso, CC BY-SA 4.0, via Wikimedia Commons. Source (right): Gabriel Barathieu, CC BY-SA 2.0, via Wikimedia Commons.

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 (Fig. 8).

Common Ancestry Fig. 8: This phylogenetic tree shows the evolutionary relationship among mammals | StudySmarterFigure 8. This phylogenetic tree shows the evolutionary relationship among mammals. Source: Graphodatsky et al., CC BY 2.0, via Wikimedia

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.

Natural selection: a process where individuals with traits that help them survive in their environment can reproduce and pass on those traits at a higher rate.

Evolution refers to a gradual and cumulative change in heritable traits of a population of organisms. This change has occurred over the course of many generations.

Common Ancestry - Key takeaways

  • Common ancestry means having descended from one ancestor.
    • Sharing a recent common ancestor means that two or more species are closely related.
    • Not having a recent common ancestor means that two or more species are distantly related.
  • 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.
    • Similarities among species are called homologies. There are three major types of homology:
      • Morphological homology: similar structure and form
      • Molecular homology: similar genes or DNA sequence
      • Developmental homology: similar developmental stages
    • Fossils show how gradual changes in the features of pre-existing organisms led to the formation of new species over time.
    • In general, the more similarities organisms share, the more closely related they likely are.
      • Most of these similarities are results of shared DNA
  • The theory of common ancestry holds that all life forms descended from one "universal common ancestor".

Common Ancestry

Structures that share a common ancestry are homologous

Common ancestry (also referred to as common descent) means having descended from one ancestor.

Similarities shared by organisms, as well as patterns in the fossil record, provide evidence of common ancestry. In general, the more similarities shared by organisms, the more closely related they likely are.

Common ancestry provides evidence for evolution because it shows that new species emerge from pre-existing species, implying that 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.

Similar traits and features among various species can provide evidence of common ancestry. In general, the more similarities shared by organisms, the more closely related they likely are. These similarities can be observed in the morphology, genes, and developmental stages of organisms. 


Fossils also show common ancestry by showing  how gradual changes in the features of pre-existing organisms led to the formation of new species over time.

Final Common Ancestry Quiz

Question

What does common ancestry mean?

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Answer

Common ancestry means having descended from one ancestor.  

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Question

What does sharing a recent common ancestor imply about the evolutionary relationship of two or more species?

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Answer

Sharing a recent common ancestor means that two or more species are closely related.

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Question

If two or more species do not have a recent common ancestor, what does it imply about their evolutionary relationship?

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Answer

Not having a recent common ancestor means that two or more species are distantly related. 

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Question

Why do we say that species are distantly related instead of unrelated?

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Answer

We say that species are distantly related because it is believed that all life forms can be traced back to one common ancestor.  

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Question

Why do similarities imply relatedness among species?

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Answer

Similarities imply relatedness because it is likely that traits and features shared by a group of organisms were inherited from a common ancestor.

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Question

Is there evidence of common ancestry?

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Answer

Similarities shared by organisms as well as patterns in the fossil record provide evidence of common ancestry. 

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Question

How do similar traits and features provide evidence of common ancestry?

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Answer

Similar traits and features among various species can provide evidence of common ancestry because it is likely that these traits and features were inherited from a common ancestor.

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Question

How does DNA provide evidence of common ancestry of life?

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Answer

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

Show question

Question

How do fossils provide evidence of common ancestry?

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Answer

Fossils show how gradual changes in the features of pre-existing organisms led to the formation of new species over time.

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Question

The theory of common ancestry holds that...

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Answer

all life forms descended from one "universal common ancestor"

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Question

When we go back in geologic time, what can we infer about the number of ancestral species?

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Answer

When we go back in geologic time, species can be traced back to a smaller and smaller group of common ancestors.

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Question

Why is genetic information important evidence of common ancestry?

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Answer

Genetic information like DNA is important evidence of common ancestry because there are instances where two or more species have major morphological differences but have nearly identical genes.

Show question

Question

How does common ancestry illustrate the unity and diversity of life?

Show answer

Answer

Common ancestry shows that new species emerge from pre-existing species and that one ancestral population could diversify into many descendant species.

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Question

How do phylogenetic trees show evolutionary relationships?

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Answer

Phylogenetic trees show evolutionary relationships by tracing different species to common ancestors.

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Question

When species are morphologically different, does it mean that they are not closely related?

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Answer

No, species could be morphologically different but genetically similar. This means that there is still a possibility that they are closely related.

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