Oh...No...No...No...This Is <em>Important</em> Spit! - Evolution News & Views

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Oh...No...No...No...This Is Important Spit!

In his recent post on Pharyngula, P.Z. Myers comments on the "breakthrough" in our understanding of human evolution that has emerged from an analysis of the genes in humans and apes that code for salivary amylase, an enzyme in spit that helps digest carbohydrates. For unfathomable reasons, this research, led by Nathaniel Dominy and George Perry, has captured quite a bit of attention in the scientific community and even in the press.

Concurrent with publication of their paper in Nature Genetics last year, Dominy and colleagues put out a press release entitled: "Extra gene copies were enough to make early humans' mouths water." Dominy bizarrely credits mutations in salivary amylase with the evolution of the human brain:

To think that world domination could have begun in the cheeks. That's one interpretation of a discovery, published online September 9 in Nature Genetics, which indicates that humans carry extra copies of the salivary amylase gene...Humans have many more copies of this gene than any of their ape relatives, the study found, and they use the copies to flood their mouths with amylase, an enzyme that digests starch. The finding bolsters the idea that starch was a crucial addition to the diet of early humans, and that natural selection favored individuals who could make more starch-digesting protein...A new ability to supplement the diet with calorie-rich starches could have fed our large brains and opened up new food supplies that fueled our unrivaled colonization of the planet, Dominy said...For Dominy and his coauthors, the finding goes beyond the mouth...anthropologists have long been stumped by the sudden, nearly simultaneous increases in our brain size, body size, and geographic range, while other apes changed little. ..."That's the big mystery of paleoanthropology," Dominy said. "What changed" Why did our earliest human ancestors deviate from the pattern we see in living apes to evolve this incredibly large brain...For years, the answer was thought to be the growing importance of meat in the diet...Some anthropologists have begun to suspect the new source of food consisted of starches, stored by plants in the form of underground tubers and bulbs-- "It's kind of a goldmine," Dominy said. "All you have to do is dig it up."...Tubers may have been especially critical for the first widely successful humans, known as Homo erectus, who may have learned to cook with fire.

Dominy and colleagues based this speculation on analysis of copy number and variation in human and ape salivary amylase, and they attribute the spit-based evolution of man to...you guessed it...natural selection! Survivors with better spit...survived, ecce homo.

I wrote a post on this Spit Brain Research, in which I pointed out that the inference from variations in an enzyme in spit to the emergence of man and the evolution of the human brain wasn't science but marketing:

The spit-brain paper no doubt contributes to the literature on salivary amylase. A study of the comparative biology of salivary enzymes -- genuine science but with limited (to say the least) popular appeal -- would have languished on dusty shelves were it not for the authors' utterly unwarranted inference that their research is relevant to the origin of the human brain. It seems to be a contemporary maxim in evolutionary biology -- 'attach preposterous speculation about the origin of the human brain to your arcane research, and you're famous,' at least for a day or two.

Nature Genetics editor Alan Packer took issue with my dismissal of the authors' linkage between salivary enzymes and the origin of man, and we exchanged posts on ENV. Dr. Packer defended the authors' evolutionary inferences.

I replied:

So are these evolutionary inferences, as distinct from the actual data on comparative genetics of salivary amylases, good science?
What if humans had low salivary amylase gene copy number, and apes had high copy number? How could the authors invoke evolutionary theory to explain these observations? The authors could assert that human vulnerability (our relative inability to digest tubers) led to a need for interdependence and socialization to ensure survival, and gave rise to human cooperation, altruism, language, and civilization. This is of course explained by natural selection.
What if humans had low intraspecies salivary amylase copy variation, and apes had high intraspecies variation? No problem. The evolutionary theory: low human intraspecies copy variation is evidence for strong evolutionary conservation -- providing further evidence that human ability to digest tubers provided enhanced energy for evolutionary adaptation or that human vulnerability (inability to digest tubers) generated a need for interdependence, gave rise to cooperation, brain growth, altruism, language, and civilization. Natural selection is consistent with either of the evolutionary hypotheses!
What if humans and apes both had high salivary amylase gene copy numbers? The evolutionary explanation: abundant salivary amylase was essential for human and ape evolution, because it allowed digestion of energy-rich tubers, thereby facilitating human and ape brain growth and giving rise to intraspecies cooperation and altruism. More evidence for natural selection!
What if humans and apes both had low salivary amylase copy numbers? The evolutionary explanation: paucity of salivary amylase is essential for human and ape evolution -- lack of ability to digest energy-rich tubers caused need for interdependence gave rise to ape cooperation, and to human brain evolution, altruism, language, civilization. Clear evidence for natural selection!
Perry et al.'s inference to natural selection is irrelevant to the specific data in their paper. 'Natural selection' could be invoked for any permutation of their data. Yet in science, inferences must depend on data, and must be subject to falsification by data. Inferences that are independent of data, such as the inference that amylase gene copy number and variation are explained by natural selection, aren't scientific inferences at all, because the inference to selection could be drawn from any data on the comparative genetics of salivary enzymes.
At the core of Darwin's theory of evolution are two hypotheses: heritable variation arose randomly, without teleology, and individuals that were rendered more reproductively successful by heritable variation were more reproductively successful. When applied to several-million-year-old genes for salivary enzymes, the first hypothesis -- that heritable variation arose without teleology -- is untested, and the second inference -- that reproductively successful individuals are reproductively successful -- is a tautology. The inference to 'evolution' in the authors' paper is an inference to the untested and to the tautological. The authors would have us believe that their inference to evolution is cutting-edge science. Yet the synthesis of 'untested' and 'tautological' isn't science at all

In his post on Pharyngula about a recent paper on this topic by Perry, P.Z. Myers, normally a dupe for any evolutionary fairy tale, seems a bit reticent to sign on completely to the spit-brain-ecce-homo breakthrough:

It implies that there may have been some selection for greater copy numbers in cultures with diets high in starchy plants... We can't entirely rule out drift as a possible cause of the difference; while we can see differences in the enzyme levels in saliva, low levels of the enzyme haven't been shown to be directly deleterious to anyone. It is again implied: oral digestion of starches may be an important pathway for taking in energy during episodes of diarrhea, so it could be critical when individuals are experiencing disease-related stress... It is very suggestive. The fact that it represents a distinct difference between other apes, such as chimpanzees, and ourselves also suggests that it may have had significant evolutionary consequences. Maybe we aren't primarily the biggest-brained apes; we are the Apes Who Eat Roots As Well As Bananas. A core nutritional difference could have played a more significant factor in our early evolution than small differences in brain size, and may have been an enabler of brain expansion.

"It implies...may have been... [w]e can't entirely rule out..[i]t is again implied...may be...could be...[i]t is very suggestive...also suggests...may have had...[m]aybe...could have...may have been an enabler of brain expansion..." Solid science, that. Here's Myers' money quote, again:

"...low levels of the enzyme haven't been shown to be directly deleterious to anyone."


Evolutionary biologists spin out this bizarre tale that natural selection acting on differences in spit played a fundamental role in the origin of man and in the evolution of the human brain, and yet, as P.Z. Myers admits, "...low levels of the enzyme haven't been shown to be directly deleterious to anyone." In other words, there isn't even real evidence that good spit confers 'selective advantage' over bad spit (carbohydrate digestion also occurs via other enzymes further along the digestive tract), let alone that it is responsible for the origin of man. This leaves a huge gap in evolutionary theory. Perhaps we are human because of better nasal mucous -- press release: "Perhaps World Domination Began in the Nostrils".

This isn't science. It's witless speculation, in service to a materialistic worldview (and grants and tenure) without a shred of relevance to our understanding of man.

And there's a deeper irony that relates to the intelligent design-Darwinism debate. Notice that the glaringly obvious I.D. inference that there is evidence for design in biology -- such as the genetic code, which has letters, words, sentences, punctuation and even syntax that are eerily similar to computer code, and the nanotechnology inside cells that is obviously a manifestation of the purposeful arrangement of parts -- is completely ruled out by Darwinists, who will go to federal court to censor even the mention of these obvious design inferences in public schools. Darwinists use legal coercion to compel teachers to remain silent about the lavish evidence for design in human biology and about the implications of that evidence for our understanding of what it means to be human. Yet schoolchildren can be taught confidently that scientists have shown that major insights into human origins has been gained by the comparative biology of spit.

Random mutations in salivary enzymes and the survival of survivors isn't the source, or even a source, of our humanity. Evolutionary biology and sociobiology entail a reduction of the salient characteristics of biology and human culture to the most rudimentary materialist speculation. It's a materialist speculative pastime, not insight of any sort. The publication in prominent scientific journals of these crudely materialistic inferences about human origins based on the comparative genetics of salivary amylase is so bizarre that it deserves nothing but scorn. It demeans science.

Here's what's important in the process by which we became human: religion, law, philosophy, literature, history, art, medicine, mathematics, natural science, psychology, just to start with. All the stuff except spit.

Anthropologist Marshall Sahlins, a harsh critic of sociobiology, has observed (*):

"...a theory ought to be judged as much by the ignorance it demands as by the knowledge it purports to afford."

Evolutionary biology is deep ignorance.

* Quoted in McKinnon, Susan; Neo-Liberal Genetics: the Myths and Moral Tales of Evolutionary Psychology, Prickly Paradigm Press, Chicago. p 149