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Homologous structures

Basic idea: In evolution, we often find similar structures in very different organisms.

Lots of creatures have wings. Why?

Lots of creatures have hands (or paws.) Why?

Possibility I

Have these animals inherited the same basic design from a common ancestor?

Are they using (roughly) the same genes, to build the same kind of structure?

Possibility II

Or have these structures independently developed from different genes? In other words, maybe it’s not from these organisms having a common ancestor.

Instead, through millions of years of trial-and-error, some basic shapes (“hands”) keep developing independently, because they are a good solution to a common problem :
(“How do we pick things up? How do we hold on to things?”)

It turns out that both types of phenomenon happen naturally through evolution.

Today we’re looking just at cases where organisms inherited the same basic design from a common ancestor. In such cases we say that they have developed homologous structures.

(On another day we will look at the other case – convergent evolution.)

History of the idea

Homology was noticed by the ancient Greek philosopher Aristotle (384–322 BCE)

He and other ancient Greeks studied anatomy, and they noticed that the bones of many very different animals were much more similar than anyone might expect them to be. Why do birds, bats, and whales all have arm structures so very much like those of humans?

Aristotle didn’t know why this was happening, but we among the first to study and describe this in detail.

Pierre Belon, in his 1555 Book of Birds, compared the skeletons of birds and humans. He noticed these homologies as well.

He didn’t know about evolution. Like many others of his time he interpreted them as part of what was called the great chain of being: the idea that life is organized on a ladder of lower to higher organisms.

Many other naturalists and scientists described many examples of homologies up through the late 1880s.

The French zoologist Etienne Geoffroy Saint-Hilaire showed in 1818 in his theorie d’analogue (“theory of homologues”) that structures were shared between fishes, reptiles, birds, and mammals.

Although Geoffroy didn’t fully the modern theory of evolution, he did see patterns and trends that didn’t match traditional views, like the great chain of being.

He came to the conclusion that species could in fact change over time.

It wasn’t until 1859 that Charles Darwin offered an explanation for how homologous structures could come into existence.

He realized that these different organisms concerned shared a body plan from a common ancestor; they inherited basic structures

and that taxa were branches of a single tree of life.

External resources



What are homographs? Homographs are words that have the same spelling, but different meanings.


What does homologous mean? It derives from the Greek ὁμόλογος homologos – from ὁμός homos “same” – and λόγος logos “plan.”

What are homologous structures? They are structures derived from a common, ancestral structure.

The limbs of humans, cats, whales and bats aren’t just similar in function and shape. They are literally built with similar bone structure, because they are programmed by the same basic set of genes.


Look at the similarities of ankle bones: dogs, ancient whales, pigs and deer.

This similarity is no coincidence – they are homologous structures.

ankle bones of dog Pakicetus (whale) pig deer

The first vertebrates on land had four limbs, with five fingers on each. Over time, vertebrates evolved into many different groups, and most of them retain homologous structures, as seen here:

adaptive radiation from five fingered hand

Analogous structures

What is an analogy? An analogy is a comparison between two objects, that highlights ways in which they are similar.

Types of analogies

The opposite of homologous structures are analogous structures.

Analogous structures look similar, and do similar jobs, but they did not evolve from a common structure – they evolved independently.

Consider the evolution of wings in bats, birds, and ancient flying reptiles. They evolved through convergent evolution.

“It is important to distinguish between different levels of homology, in order to make informative biological comparisons: bird and bat wings are analogous as wings, but homologous as forelimbs. Why? Because the organ served as a forearm (not a wing) in the last common ancestor of tetrapods.” {http://en.wikipedia.org/wiki/Homology_(biology)

Homologies from Understanding Evolution

Images from

1 Comment

  1. […] Similar structures like the one described above can be found throughout various different species for many millions of years. Here are some links to get you started. […]


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