High five!
Q:
Why do we have five fingers? (Faisal, World)
 A:
It's not just us. Mammals, birds, and reptiles--all have no more than five digits in their forearms.
High
five. Drawing courtesy of the US Forest Service.
We have five fingers because our ancestors had five digits-going back at least to a mouse-sized, lizard-like beast, named
Casineria kiddi, who scampered around lagoons, 340 million years ago.
Genes have carried the information from those ancient to these modern times. An intriguing combination of genes control
digit development in embryos. These tiny spots on wispy bits of DNA are called Hox genes and specify how an embryo
builds its digits-and its limbs, spine, gut, urinary tract, and genitals-vital organs all.
The 39 Hox genes are arranged into four strings. The genes in each string turn ON and OFF in sequence as the embryo grows. The time a Hox gene is ON
makes a difference. Swans, for example, have more neck vertebrae (and consequently, a longer neck) than chickens because their neck-bone Hox genes stay ON
longer.
Perhaps the fin Hox genes of an ancient fish stayed ON longer so instead of fins, the fish sprouted hands. Something like that happened 360 million years ago
to evolve the 3-foot long salamander look-alike, named Acanthostega. Figure 2. This creature had hands, feet, and legs, although he spent almost his entire life
in the water. Curiously, his appendages had eight digits (figure insert).
 No land vertebrate has had more than five digits for the past 340 million years when the Casineria first scuttled on its
5-digit feet. Why eight for these earlier animals?
Acanthostega,
Photo courtesy of Jennifer Clack, model by Richard Hammond.
"The larger number of digits fits the hypothesis that early in limb evolution, digit number was not fixed," says Jenny
Clack, who with Michael Coates discovered complete specimens of Acanthostega in Greenland.
Acanthostega used their limbs to crawl and paddle around shallow waters. Eight fanned-out digits help paddling but not walking. Land walkers discarded the
extra digits and may have fixed the number at five.
Remember, Hox genes control, not only digit number, but also the urogenital organs. The same HOXA13 genes control human hand, foot, and genital
development. If Nature tinkers with digits, she may mess up reproduction. Too dangerous.
We have five fingers, therefore, because we don't dare try anything else. On the other hand...
"Because it hasn't been selected against, yet. Think long term! Says Jeff Innis, University of Michigan pediatric geneticist.
(Answered by April Holladay, science correspondent, May 23, 2001)
Further Surfing:
U of Ariz: Origin of limbs with digits-Acanthostega
Time Magazine: Where do toes come from?
Science News Online: Out of the swamps
Hox Pro db: Hox affects limbs and genitals
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