On Homology

Richard Owen, one of the great fathers of paleontology, is a curious figure in the history of biology. A contemporary of Darwin’s, he had ambiguous and sometimes self-contradictory views of evolution. Nevertheless, his most lasting contribution to modern biology is the concept of homology, one of the guiding principles of evolutionary biology.

Owen noticed that the limbs (for example) of bats, cats and seals all had a common plan – a single humerus followed by a pair of parallel bones called the radius and ulna, two rows of pebble-like bones called carpals and then a system of  long, thin bones called metacarpals and phalanges – even though they had very different functions and outward appearances. As a result, he drew a distinction between two different kinds of anatomical similarity. When two organs are similar only in function, but not in appearance, he said that their relationship was one of “analogy.” But when two organs were similar because they fit into the same plan, he called the relationship “homology.” The bat wing, cat paw and seal flipper are homologous with each other, but a bird’s wing and a fly’s wing are analogous.


The limbs of a human, dog bird and whale, color coded to show homologous bones. Courtesy Wikimedia Commons.

Oddly, although he knew Darwin’s work very well, and although the concept of homology is now explained in terms of evolutionary relationships, Owen maintained that homology only reflected the common designs in the mind of the Creator. Today, we have a different view: homology is defined as similarity as a result of separate descent from a common ancestor. The bat, the cat and the seal all had a common ancestor whose arm-bones were laid out in the same way as its descendants’.

The concept of homology is both crucial to evolutionary biology and maddeningly ambiguous (somewhat like Owen himself). It is crucial because if we want to ask essential questions such as, “how did birds evolve wings?” we must necessarily talk about homologous structures in the ancestors of the birds. But it is ambiguous for a host of practical and philosophical reasons.

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Idle Musings on Evolutionary Time

Since Aristotle, students of nature have been tempted to rank some organisms as somehow “better” than others. Aristotle ranked all organisms from most simple to most complex. In Medieval Europe, his ideas were taken up and incorporated into a grand scala naturae or ladder of life, with lowly worms at the bottom, humans at the top of the mortal beings, and angels above us.

Evolutionary theory has had its share of attempts to understand the scala naturae, usually with time playing the role of the force that makes some organisms more “evolved” than others. Lamarck posited multiple origins of life over the ages and suggested that the lowliest species are newcomers on the world’s stage, whereas loftier species had been around for longer and attained greater heights. Hints of this view still resonate in popular misconceptions about evolution.

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Of Frog Eyes and Jellyfish Tentacles

Author’s note: There were errors in the original version of this article. Please see the Post Scripts for more details.

In 1901, Hans Spemann revolutionized biology by doing something very strange. He had been watching various embryos grow, and he got bored. That’s the short version of his motivation.

Basically, the embryologists of the day had already spent oodles of time carefully documenting the normal development of various animals from egg to embryo to hatchling. They had established that most animals – vertebrates included – go through a succession of embryological stages called the morula, blastula and gastrula. The morula is just a dense ball of cells (In Latin, morula means “mullberry”), the blastula is a hollow ball of cells, and the gastrula is like a blastula with an indentation somewhere. The indentation keeps growing inward until it meets the other side and becomes a tube that runs through the embryo’s whole body. This tube becomes the digestive tract, which, if you think about it, is just a tube running through an animal’s whole body. In some species, called the protostomes, the original indentation becomes the mouth, but in other animals (deuterostomes) it becomes the anus. We are deuterostomes. In fact, it’s all very interesting, because the same program seems to occur in wildly different organisms, from worms to molluscs to starfish and humans.

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The Great Eukaryotic Melee

Cat fight! Mrowwwr! Hisssssss!

Every now and then, you can capture a snapshot of the scientific process at its worst (and paradoxically, its best, too). The trick is to look at the Letters or Perspectives section of your favorite scientific journal.

In this case, I happened to come across an argument between two sides in what I will call the Great Eukaryotic Melee (there are probably more than just two sides in this debate, but only two are reflected on these pages). Actually, the debaters seem to be rather more well-behaved than I suggested above, but it is true that these fights do sometimes turn ugly.

The central issue? How did Eukaryotes evolve. Let’s review some basics.

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Watching the Watchmaker

There is an old argument that creationists like to trundle out from time to time, called the Watchmaker Argument. It’s been roundly defeated by evolutionary biologists (many times over), but I’d like to address a part of it that most modern evolutionists skip over, because it’s an interesting philosophical exercise.

In 1802, William Paley published, Natural Theology, which was an extended argument in favor of the existence of God, and a refutation of the current ideas about evolution. Astute readers might have noticed that 1802 was 57 years before the publication of On the Origin of Species by Charles Darwin. In fact, Darwin took the arguments in Natural Theology very seriously and went to great lengths to refute them. I suppose he might have had it in front of him while he composed his treatise, although I’m not enough of a historian to say.

Anyway, the central argument of Natural Theology goes like this. Say you’re walking along, thinking evolutionary thoughts to yourself, when you come upon a pocket watch. The watch was probably planted there by a creationist, but that’s neither here nor there. Anyway, you pick up the watch and examine it, and immediately understand that it is a complex device that was been made by a watchmaker. It did not just pop up out of the ground like a stone.

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Anolis urbis

Move over, moths! There’s a new kid in town!

During the Industrial Revolution, naturalists in England noticed that the incidence of normal, light-colored peppered moths (Biston betulari) had become scarce in the vicinity of various urban centers. Instead, they were finding a melanistic (dark colored) variety. At the same, time, pollution had caused the local trees to get darker. In 1896, J.W. Tutt proposed that this change was an example of natural selection. The light moths lost their camouflage effect when they sat on the new dark trees, so they got eaten by the local birds. Hence, the moths with the gene for melanism fared better and became prevalent.

The issue was hotly debated and thoroughly investigated through the first half of the twentieth century, and has since become one of the best known and best supported examples of natural selection. You probably remember it from your high school biology class.

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Bracketing the Past

How do we learn about extinct things? Can we use evolutionary theory itself to help us? Yes, but first we need to take a heuristic detour into space.

Suppose you’re an alien from another star system, maybe a thousand light-years from ours. Your scientists pick up radio wave transmissions from Earth, but they are garbled. You know there’s a civilization here, and you can figure that the transmissions came from the third planet, but you don’t have much more detail yet. So, you pack up your spaceship and head over our way.

Unfortunately, in the intervening time, us silly humans manage to blow up the Earth. When you arrive in the Sol system and come out of cryosleep, all that’s left here is a shiny new ring of asteroids where our big blue marble used to be. (Sigh …it happens.)

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