SCIENCE OBSERVER
Descending Scales
Greg Ross
Snakes have been around for at least 100 million years, but it's
still not clear whose family reunion they should go to. Biologists
agree that they descended from lizards, but exactly where this first
happened has been a matter of debate since the 1800s, when two
contending theories emerged.
The so-called terrestrial hypothesis holds that snakes evolved from
land lizards that shed their limbs. The marine hypothesis contends
that they appeared first at sea and slithered ashore later.
Which is correct? In recent years, the marine hypothesis found some
support when a group of extinct marine snakes from the Cretaceous
were discovered to have vestigial limbs. But a new study using
genetic analysis comes down firmly on the landlubbers' side.
By scrutinizing evolutionary changes in two genes, biologists
Nicolas Vidal and S. Blair Hedges of the Pennsylvania State
University set out to construct a family tree for modern snakes and
lizards. The team collected data from 64 different species, going to
extra lengths to get tissue samples for all 19 families of living
lizards, including some rare specimens from Africa, China and the Philippines.
"The importance of having all the families of lizards is that
we simply don't know the closest family of lizards to snakes,"
Hedges says. "So if you're missing one or two, that could be
the one. You really have to have all of them."
If the terrestrial hypothesis is correct, today's snakes are most
closely related to land lizards. If the marine hypothesis is
correct, modern snakes can claim a closer genetic cousin in the sea.
(When snakes first appeared, the only seagoing lizards were giant
mosasaurs. The mosasaurs left no tissue samples, so Hedges and Vidal
used their closest living relatives, monitor lizards, as genetic stand-ins.)
After compiling gene sequences from each species, the team compared
them using a variety of statistical methods, looking for patterns of
mutation. Their findings, published in the U.K. Royal Society
journal Biology Letters, suggest that snakes are not
closely related to monitor lizards—or, hence, to ancient sea
lizards. Because all other lizards at that time lived on land, the
Penn State study supports the idea that land lizards evolved into
modern snakes.
"The tree unfortunately doesn't show exactly what lizard family
is the closest relative of snakes significantly," Hedges says.
"But what it does show significantly is that the closest
relative is not the monitor lizards, the varanids, which is the only
connection that would have supported the marine hypothesis."
The terrestrial hypothesis makes intuitive sense to Hedges, who used
to explore caves in high school. "The first thing you realize
is that your limbs are a detriment to moving through small
openings," he says. "So it's very easy and logical to
understand why natural selection would result in a limbless organism."
In fact, "we see this in many groups of lizards that have small
limbs or completely missing limbs, reduced to where there are no
limbs externally visible. Quite often those lizards burrow in the
ground or live in holes. So it's very logical that the ancestor of
snakes might have been a burrowing sort of lizard."
One that learned to walk before it could crawl.