What are structures that have a common function and suggest common ancestry?

By Sam Lucas

Inhaltsverzeichnis Show

Homologous Structures Definition
Examples of Homologous Structures
A Tale of Tails
Eye Have a Light Bulb
Raise Your Head High
What is an example of common ancestry?
What provides common ancestry of all life?

Comparative anatomy is the study of similarities and differences within the structures of organisms. Physical features may be considered homologous or analogous, but what does this mean?

Homologous structures are similar physical features in organisms that share a common ancestor, but the features serve completely different functions. An example of homologous structures are the limbs of humans, cats, whales, and bats. Regardless of whether it is an arm, leg, flipper or wing, these structures are built upon the same bone structure.

Diagram from: https://www.ck12.org/book/cbse_biology_book_class_xii/section/9.9/

Homologies are the result of divergent evolution. Divergent evolution is the process in which organisms from the same common ancestor evolve and accumulate differences, often resulting in a new species. This may occur due to pressures such as changes in abiotic or biotic factors within the environment.

On the opposite side of the spectrum, analogous structures are similar physical features in organisms that do not share a common ancestor. Instead, these structures are related to one another because they perform the same function. An example of this are the wings of a bat and the wings of a bird. They have completely different bone structure, but their wings share the same function, allowing the animal to take flight.

Diagram from: https://evolution.berkeley.edu/evolibrary/article/_0_0/evo_09

Analogies are a result of convergent evolution. Convergent evolution is the process in which two organisms that do not share a common ancestor evolve and develop similarities independently of one another. These similarities form because the animals either live in comparable environments or they experience the same environmental pressures, resulting in the evolution of these features.

Next time you are comparing two organisms try to think about how they are related (or not) to one another. The answers may surprise you!

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Homologous Structures Definition

Homologous structures are organs or skeletal elements of animals and organisms that, by virtue of their similarity, suggest their connection to a common ancestor. These structures do not have to look exactly the same, or have the same function. The most important part, as hinted by their name, is that they are structurally similar.

Examples of Homologous Structures

A Tale of Tails

Monkeys, cats, rats and other mammals have tails. In mammals, the tail is an extension of the torso, made of flexible vertebrae. Tails primarily function to ward off insects, but they can also serve as sources of balance for more aloof species, like cats.

Humans possess a similar feature known as the coxxyx, or tailbone. Also an extension of the torso, it is made of what some scientists call “rudimentary vertebrae” and is thought to have once been a fully-formed tail. Unlike other mammal tails, however, it currently serves no purpose.

The fact that the structure of the human coccyx so closely resembles that of an animal tail gives scientists reason to link it to a common ancestor between mammals and humans. Due to this link, the mammalian tail and the human coccyx are homologous structures.

Eye Have a Light Bulb

Not all animals can see the way humans do. Deep sea creatures, like the chimera, live in an environment so dark, their eyes have not developed the sophisticated discriminatory skills that have, human eyes. Their visual cues come from light receptors near the front of their skull, and they do not see color or depth.

Like light receptors, the human eye catches light and sends that information to the brain. As we humans have evolved in an environment fully illuminated by the sun, their eyes contain extra filters, or photoreceptors, that allow us to perceive colors, shadow, and distance. More specifically, rod-shaped photoreceptors allow us to see black-and-white and shadow, and cone-shaped photoreceptors allow us to see color and saturation.

The image shows a blind chimera that “sees” with light receptors.

The image shows a human eye uses rods and cones to convert light into images.

Thanks to evolution and environment, the function of the human eye developed into something far more sophisticated than that of deep-sea creatures, like the chimera. However, despite our ability to see full images and the chimera being restricted to only shadow, the fact that both eyes and light receptors “see” by taking in light confirms there possible connection to a common ancestor and, therefore, structurally homologous.

Raise Your Head High

Giraffes are the subject of wonder and amazement, and rightly so. Since Carl Linnaeus first classified them in 1758, these animals have captured the eye of all who explore the Sahara.

Their long necks, especially, gather the bulk of attention. Although they measure up to eight feet in length and weigh over 600 pounds, they contain only seven cervical vertebrae, or neck bones. Looking at the image below, we see that these bones tend to be longer – about one foot in length.

Humans also have cervical vertebrae, but they tend to be far shorter than those of the giraffe. Examining the human neck diagram, human cervical vertebrae appear much shorter and squatter than those of the giraffe.

A giraffe’s long neck

Nonetheless, both human and giraffe necks contain seven bones. This number, when combined with the similar structure of the human and giraffe spine, allows the scientific community to posit that humans and giraffes, as different as they may be, share a common ancestor. As such, their cervical vertebrae are structurally homologous.

Coccyx – The “tail bone” at the end of a mammal’s spinal column. The coccyx may be composed of fused vertebrae, or it may extend into a tail.
Photoreceptor – A structure, usually a cell or small organ, which detects any light that falls on it.
Cervical vertebrae – The bones that provide support for the upper spine.

Quiz

1. Homologous structures do not have to have the same function. Instead, they must:
A. Link both species to a common ancestor.
B. Look exactly the same.
C. Move in the same direction.
D. Follow similar growth patterns.

Answer to Question #1

A is correct. Homologous structures must be similar enough that they suggest that two species share a common ancestor. However, they do not have to serve the same function.

2. Only mammals can share homologous structures.
A. True
B. False

Answer to Question #2

False. While mammals share a number of homologous structures, homologous structures are not shared exclusively between mammals. Human eyes, for instance, are homologous to photoreceptors found in deep-sea creatures.

3. Human photoreceptors are different from chimera photoreceptors because:
A. They are better-suited for dark environments.
B. They get better reception.
C. Chimeras cannot see.
D. They contain rods and cones that interpret the image created by light information.

Answer to Question #3

D is correct. Human eyes contain rods and cones, which allow them to see color and depth more clearly. Since chimeras live in a dark environment, their eyes have only evolved to the degree that they can detect shadow.

What is an example of common ancestry?

Ancestral organism shared by two or more descendent lineages — in other words, an ancestor that they have in common. For example, the common ancestors of two biological siblings include their parents and grandparents; the common ancestors of a coyote and a wolf include the first canine and the first mammal.

What provides common ancestry of all life?

Answer and Explanation: The best evidence for the common ancestry of all life on Earth is A) universality of the genetic code. All living organisms have a genetic code composed of the same four nucleotides and often make use of the same (or very similar) enzymes to perform the same metabolic processes.