|Evolution in the News - June 2005|
|by Do-While Jones|
“Our ideas about how crocodiles evolved have just taken a battering. It seems that these cold-blooded creatures, with their limited capacity for prolonged activity, might have had active, warm-blooded ancestors.” 1
Fish have a two-chambered heart which pumps blood in a single loop through the gills and body before returning to the heart. Mammals have a four-chambered heart. Two of those chambers pump blood (at a low pressure) through a loop which goes through the lungs back to the other side of the heart. The other two chambers pump blood (at a high pressure) through a second loop that goes through the body and back to the heart. Reptiles have a three-chambered heart that takes blood from the lungs and the body, mixes it together in the heart, and pumps the mixture to the lungs and the body.
This miraculous evolution in plumbing, evolutionists say, is the result of fortunate mistakes in reproduction which caused more heart chambers and extra circulatory loops. You could say the mammal heart arose because of “a reptile dysfunction” (but be careful if you say that out loud ).
Warm-blooded, or 'endothermic', vertebrates (birds and mammals) have high resting metabolic rates, enabling them to maintain a constant body temperature that is usually above that of their environment. Endotherms use more oxygen and need more fuel than cold-blooded 'ectothermic' vertebrates, such as fish, lizards and frogs, with their lower and more variable body temperatures. But, as a consequence, endotherms have gained a capacity for prolonged strenuous activity that can't be matched by ectotherms.
Birds and mammals have independently evolved a four-chambered heart divided into two sides, so that the oxygenated blood coming from the lungs is separated from the deoxygenated blood arriving from the rest of the body. 2
Notice that it is simply stated without proof that birds and mammals evolved four-chambered hearts independently. There is no biological evidence to support this claim that the miraculous mistake happened twice. It is simply based on the dubious assumption that heart evolution happened after the unknown bird and mammal ancestors split from reptiles. Even evolutionists recognize this as a problem.
A muscular ridge in the three-chambered heart can completely separate oxygenated from deoxygenated blood, so mere separation of blood flow does not explain why a four-chambered heart is so beneficial that it evolved twice. 3
So, the newly proposed fairy tale is that the four-chambered heart evolved once in ancient land-dwelling crocodiles, then evolved back to what is essentially a three-chambered heart when crocodiles started living in the water. Here is the current (April 2005) fable:
Because heat loss in water can be many times higher than that in air, and because larger animals have higher metabolic rates, there would have been considerable selective pressure against endothermy during this return to the water. … Seymour et al. propose that the ancestors of modern alligators and crocodiles were endothermic and required a four-chambered heart for pressure separation, and that it has since re-evolved a shunting system. 4
The motivation for this speculation comes back to the dinosaur problem. When I was a little boy who knew everything there was to know about dinosaurs, I knew that dinosaurs were cold-blooded lizards. Now, 50 years later, children are being taught that dinosaurs were warm-blooded birds. (This is why the subtitle of the “Dinosaurs—Ancient Fossils, New Discoveries” exhibit, now through January 8, 2006 at the American Museum of Natural History, is “What you thought you knew is history.”) But there were good reasons why scientists used to believe dinosaurs were cold-blooded, and the question isn’t as well-settled as the general public thinks.
There is little agreement among experts about the body temperature of dinosaurs, and without significant new fossils direct evidence is unlikely. That leaves the question in the camp of comparative physiologists who study the working of extant animals in hopes of deducing the function of animals long since turned to dust. Evidence from birds and crocodiles, the two living groups that 'bracket' the dinosaurs, is particularly useful. If ancestral crocs had high resting metabolic rates, the clear implication is that dinosaurs did as well. 5
If dinosaurs were warm-blooded, then it removes the difficult question of how cold-blooded dinosaurs could have evolved into warm-blooded birds. But that just moves the problem back farther into evolutionary history. How did cold-blooded reptiles evolve into warm-blooded reptiles (like the supposedly warm-blooded crocodiles proposed by Seymour)? Why aren’t there any warm-blooded reptiles today?
It also brings back the two big thermodynamic problems that caused scientists in the 1950’s to conclude that dinosaurs must have been cold blooded. First, how could an animal with a small surface-to-volume ratio shed all the heat that would be produced by endothermic metabolism? Second, how could such a big animal get enough food to sustain endothermic metabolism? This is why “there is little agreement among experts about the body temperature of dinosaurs.”
But let’s not get distracted by these things. The real questions are how hearts can add or lose chambers, how new circulation loops can appear, why metabolism would change, how metabolism could change, etc.
The idea that “heat loss in water” would have caused “considerable selective pressure” is simplistic. It ignores the problem of creating things to select from. Did the water (that is, the environment) cause the offspring to have different hearts than their parents? There is no scientific evidence to suggest that water has that power.
Was it just good luck that some creatures were born with different hearts and different metabolism at the same time? We don’t see that kind of good luck happening with living animals. Baby critters generally have the same number of chambers in their hearts as their parents do. On those rare occasions when they don’t, the birth defect generally causes them to die.
It isn’t just that “Our ideas about how crocodiles evolved have just taken a battering.” 6 The theory of evolution is taking a scientific battering every day.
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Sumers, Nature, 14 April 2005, “Warm-hearted crocs” page 833