Atavism

[frogmarch]

from Why Evolution is True by Jerry A. Coyne: http://jerrycoyne.uchicago.edu/about.html To read the blog, click blog in the left sidebar.

One of my favorite cases of embryological evidence for evolution is the furry human fetus. We are famously known as "naked apes" because, unlike other primates, we don't have a thick coat of hair. But in fact for one brief period we do--as embryos. Around sixth months after conception, we become completely covered with a fine, downy coat of hair called lanugo. Lanugo is usually shed about a month before birth, when it's replaced by the more sparsely distributed hair with which we're born. ... Now, there's no need for a human embryo to have a transitory coat of hair. After all, it's a cozy 98.6 degrees Fahrenheit in the womb. Lanugo can be explained only as a remnant of our primate ancestry: fetal monkeys also develop a coat of hair at about the same stage of development. Their hair, however, doesn't fall out, but hangs on to become the adult coat. And, like humans, fetal whales also have lanugo, a remnant of when their ancestors lived on land.

Wiki:

The term atavism (derived from the Latin atavus, a great-grandfather's grandfather; more generally, an ancestor) denotes the tendency to revert to ancestral type. An atavism is an evolutionary throwback, such as traits reappearing which had disappeared generations ago. Atavisms occur because genes for previously existing phenotypical features are often preserved in DNA, even though the genes are not expressed in some or most of the organisms possessing them.

My new adoptive grandson, born almost a week ago, entered the world with baby "down," sometimes called lanugo. I wondered if this temporary body and facial fuzz that some human babies are born with, might be an atavism. (Apparently some biologists, including Jerry A. Coyne, think it is.) This led me to wonder about other unusual features some of us humans exhibit, such as extra nipples and so forth.

How are biologists able to distinguish an atavism from a non-atavistic mutation?

[mike_c]

1. the usual way is via phylogeny....

First off though, it is ALWAYS conceivable that a trait can arise through mutation multiple times during a lineage's evolutionary history, but if an ancestral taxon has the trait, especially a closely related ancestor, then the most parsimonious explanation is usually atavism.

[frogmarch]

4. Thanks. That's a clear, concise answer!

The more I read on the topic, the more complex I realize it is. It's fascinating!

[Warpy]

2. Ontogeny recapitulates phylogeny

We also start out with gills early in gestation. There is as little reason for gills as there is for lanugo, but the expression of our evolution from sea to land is there, too.

[frogmarch]

5. The subject, although fascinating,

is confusing to me

I've been reading online about atavism, which led me to articles about vestigial structures. Here's an article about vestigial legs that were found on a young shark:

http://www.underwatertimes.com/news.php?article_id=73210864950





Sharks didn't evolve from land animals, so how could a shark have legs, unless the presence of legs on this shark is a freak mutation and not an atavism? If not an atavism, the legs wouldn't be vestigial, right?

Could this story be a hoax?

EDIT: Those have got to be CLASPERS! (I feel so stupid!) :eyes:

[onager]

6. Another shark mutation!

Discovery-Science channel recently ran a show called "Radioactive Paradise." About a modern research team studying the effects of nuclear testing at Bikini Atoll in the 1940's and Fifties.

The nurse sharks in that area usually come equipped with 2 dorsal fins. The researchers had heard that some nurse sharks were showiing a mutation - they had lost the second dorsal fin and only had a small bump where the fin used to be.

Sure enough, they found one of those sharks and filmed it, proving that the sharks were passing on the mutation.

Definitely worth catching if you can. The show, not the shark, that is.

[PetrusMonsFormicarum]

3. Thanks!

The drummer in my band is a hairy little sonofabitch, and we've been looking for a new nickname for him. LANUGO it is!