Jun
17
2006

Solving the mystery of whales' missing legs

Solving the mystery of whales' missing legs: Credit: Carole Harwood/NEOUCOM  Solving the mystery of whales' missing legs
Solving the mystery of whales' missing legs: Credit: Carole Harwood/NEOUCOM Solving the mystery of whales' missing legs

Every week, as I stand under the Science Museum of Minnesota's whale skeleton, I wonder if there is any remnant of its back legs and pelvis? I learned in a recent exhibit that whales once lived on land - they actually share a common ancestor with hippos, camels and deer.
A recent paper by J.G.M. 'Hans' Thewissen in the Proceedings of the National Academy of Sciences helped me to understand what might have happened.
One way to figure out evolution is to watch embrionic development. By studying hindlimb development in dolphin embryos, Thewissen theorised that "The presence of the initiation of hindlimb development suggests that dolphins had terrestrial ancestors with four limbs." Several of these ancestors have been found as fossils.

In most mammals, explains Thewissen, "a series of genes is at work at different times, delicately interacting to form a limb with muscles, bones, and skin. The genes are similar to the runners in a complex relay race, where a new runner cannot start without receiving a sign from a previous runner."

In dolphins, however, at least one of the genes drops out early in the race, disrupting the genes that were about to follow it. That causes the entire relay to collapse, ultimately leading to the regression of the animals' hind limbs. By analyzing dolphin embryos, Thewissen showed that the dropout is a gene called "Sonic Hedgehog," which is important at several stages of limb formation.

In whales, however, the story is more complex. Between 41 million and 50 million years ago, whales' hind limbs did shrink greatly as the former land animals began a return to the sea. But their legs showed no change in the basic arrangement and number of bones, which proved that Sonic Hedgehog was still functioning. Its loss must have come later. In short, "the dramatic loss of Sonic Hedgehog expression was not the genetic change that drove hind limb loss in whales".

Instead, Thewissen and his colleagues conclude, whales' hind limbs regressed over millions of years via "Darwinian microevolution": a step-by-step process occurring through small changes in a number of genes relatively late in development.

Abstract from PNAS: Developmental basis for hind-limb loss in dolphins and origin of the cetacean bodyplan

Among mammals, modern cetaceans (whales, dolphins, and porpoises) are unusual in the absence of hind limbs. However, cetacean embryos do initiate hind-limb bud development. In dolphins, the bud arrests and degenerates around the fifth gestational week. Initial limb outgrowth in amniotes is maintained by two signaling centers, the apical ectodermal ridge (AER) and the zone of polarizing activity (ZPA). Our data indicate that the cetacean hind-limb bud forms an AER and that this structure expresses Fgf8 initially, but that neither the AER nor Fgf8 expression is maintained. Moreover, Sonic hedgehog (Shh), which mediates the signaling activity of the ZPA, is absent from the dolphin hind-limb bud. We find that failure to establish a ZPA is associated with the absence of Hand2, an upstream regulator of Shh. Interpreting our results in the context of both the cetacean fossil record and the known functions of Shh suggests that reduction of Shh expression may have occurred {approx}41 million years ago and led to the loss of distal limb elements. The total loss of Shh expression may account for the further loss of hind-limb elements that occurred near the origin of the modern suborders of cetaceans {approx}34 million years ago. Integration of paleontological and developmental data suggests that hind-limb size was reduced by gradually operating microevolutionary changes. Long after locomotor function was totally lost, modulation of developmental control genes eliminated most of the hind-limb skeleton. Hence, macroevolutionary changes in gene expression did not drive the initial reduction in hind-limb size.

photos and comments on whale limb rudiments
NSF press release
EurekaAlert

Your Comments, Thoughts, Questions, Ideas

Anonymous's picture
Anonymous says:

i never knew whales had legs. since when do they?

posted on Wed, 08/09/2006 - 10:06am
Anonymous's picture
Anonymous says:

WHAT IS COMMON AMONG MAMMALS ARE THE BEGINNING OF EMBRYONIC DEVELOPMENT. WHAT MAKE EACH SPECIES DIFFERENT IS 1] THE NUMBER OF CHROMOSOMES (DNA) AND 2] CELL DIFFERENTIATION. CELL DIFFERENTIATION DISCONTINUES BUD DEVEOPMENT FOR HIND LIMBS.

posted on Sat, 01/15/2011 - 1:51pm
ARTiFactor's picture
ARTiFactor says:

Good question. Whales are descendants of land-living mammals. Read about the evolution of whales here.

posted on Fri, 08/11/2006 - 9:47am
tommy's picture
tommy says:

i toght that whales could walk on their legs

posted on Mon, 02/26/2007 - 2:46am

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