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Monotremes: Bird-mammal hybrids?

One strong clue that platypuses may be bird-mammal hybrids is the fact that in research appearing in the journal Nature Grützner et al. (2004) reported that platypuses, despite being classified as mammals, lack the gene SRY, which determines sex in mammals, and instead have a gene similar to DMRT1, which determines sex in birds (see also: Veyrunes et al. 2008). They demonstrated that the platypus genome contains sex chromosomes of both bird and mammal origin (read an article about this research >>). As Grützner, specifically stated in an interview: “The platypus actually links the bird sex chromosomes system with the mammalian sex chromosome systems.”

A draft version of the platypus genome (Warren et al. 2008) identified at least two genes otherwise known only in birds (see also Rens et al. 2007). Indeed, some of the sex chromosomes of monotremes share homology with bird sex chromosomes rather than those of mammals, report Veyrunes et al. (2008), who state that there is no homology between the platypus and therian X chromosomes…the platypus X chromosomes have substantial homology with the bird Z chromosome (including DMRT1) and to segments syntenic with this region in the human genome. Thus, platypus sex chromosomes have strong homology with bird, but not to therian† sex chromosomes.

Dohm et al. (2007) and Rens et al. (2007) have also identified regions of homology with the chicken Z sex chromosome on the platypus sex chromosomes X2p and X1p. And yet, Veyrunes et al. (2008, p. 966) found that "not a single BAC identified on the platypus sex chromosomes represented scaffolds that shared homology with the human X chromosome." In other words, researchers have found large regions of the platypus sex chromosomes that are very similar to regions on the chicken sex chromosome Z, and yet no similarity whatsoever to the sex chromosome of a mammal, i.e., the human X chromosome. And Veyrunes et al. (2008, p. 970) specifically state that "Platypus sex chromosomes are more bird-like than mammal-like."

The foregoing genetic evidence, then, is clearly consistent with the idea that platypuses are anciently derived from a cross between a mammal and a bird. So is the fact that they both lay eggs and produce milk. Their strange morphology, too, combines birdlike traits with those of a mammal. Obvious and well-known features of this sort are the presence of both a duckbill and a hair coat. But they also have a variety of lesser-known traits that seem to connect them with birds despite their classification as mammals.

If platypuses are not bird-mammal hybrids, then why should this be the case? It seems that only an extreme bias against the idea that such distant hybrids might be possible can account for the failure of scientists to recognize this obvious implication.

Available evidence for echidnas, the other type of monotreme, points in the same direction as what's available for platypuses. Like platypuses, echidnas lay eggs and produce milk. They also have a birdlike beak, but lack the feathers of a bird, and instead have a coat of spines like a hedgehog or porcupine. And they have many other avian characteristics.

The name monotreme, meaning “single orifice,” refers to the presence in both platypuses and echidnas of a cloaca, a common chamber into which the intestines, as well as the reproductive and urinary ducts, vent (Vaughan et al. 2011, p. 120). Though monotremes of both sexes do have a cloaca (as do birds, reptiles and amphibians), other mammals don't—with the exception of certain Malagasy insectivores known as tenrecs (Vaughan et al. 2011). In fact, the great French anatomist Georges Cuvier stated that the reproductive tracts of monotremes are “infinitely closer to those of oviparous animals than to those of mammals” (Recherches sur les ossemens fossile, vol. 5, p. 68).

It is because platypuses and echidnas have many features that are typical of birds, and not of mammals, that they are classified as separate from all other mammals. Since their genomes both contain genes otherwise found only in birds, and since they have so many other traits that connect them with birds, is it not, perhaps, reasonable to consider the possibility that they are not merely birdlike mammals, but that they are actually rare examples of hybrids descended from ancient, unusual matings between mammals and birds?

One might suppose that such distant crosses would be absolutely impossible, which is the big sticking point for some people. Indeed, there are those who are quite vehement on this point, though it's hard to see why, since little experimentation has been carried out to test the question. Commercial investigations using artificial insemination demonstrate beyond doubt that in some fairly distant crosses among birds (e.g., turkey × chicken or quail × chicken), a small percentage of fertilized eggs actually develop into mature hybrids (McCarthy 2006). And extensive experiments involving the hybridization of distantly related fish, Newman (1915, 1917 and 1918) found that many such crosses did produce hybrids, even in the case of inter-ordinal crosses.

And such findings do not go entirely against expectation. After all, it's not as if a mammal spermatozoon that suddenly found itself in the cloaca of a bird would clasp its blushing cheeks and cry, “Oh my ears and whiskers! I should not be here! Whatever shall I do?” On the contrary, one might reasonably suppose that, at least in certain instances, any such mislaid gamete might swim merrily on, and that one or more of them might at last reach an ovum, and that sometimes, perhaps very rarely, fertilization might occur. And over geological time, among the many such zygotes formed, some few might well have reached maturity and produced offspring.

So it may well be that even in very distant crosses, such as one between a bird and a mammal, after many thousands of matings, a few hybrids would be produced. Indeed, by following the links in the sidebar at right, you will find that there is a great deal of evidence supporting the idea that such hybrids actually do occur. Of course, without experimentation the frequency and ease with which such hybrids are produced must remain a matter of speculation.

If platypuses and echidnas were bird-mammal hybrids it would explain why they, unlike other mammals have so many avian traits. As has already been mentioned, such traits are enumerated on a separate page of this website. As to the specific type of mammal and bird proposed as parents, for a platypus a likely choice would seem to be some sort of duck mated with an otter- or perhaps beaver-like mammal. In the case of the echidna, a reasonable combination would be a hedgehog mated with a kiwi (an echidna actually looks much like a hedgehog body with a kiwi head attached). Such crosses would, of course, be far outside what conventional biological thought would accept as possible. But, then again, perhaps convention is mistaken.

Creativity, Genes, and Racial Differences

Davide Piffer is an Italian evolutionary anthropologist. He received a BA in Anthropology from the University of Bologna and a Master of Science in Evolutionary Anthropology from Durham University in England. His Master’s thesis was on the sexual selection of sleep patterns among humans, and was the first to link mating behavior to chronotype (sleep patterns during a 24-hour period) within an evolutionary framework. His research later shifted to quantitative genetics, and he published one of the first studies of the heritability of creative achievement. In 2013, he switched to molecular genetics, focusing on the polygenic evolution of educational achievement and intelligence, and this remains his main focus. Within this area, his main finding is that ethnic differences in intelligence are largely explained by the thousands of genetic variants that predict cognitive abilities within populations.

Grégoire Canlorbe: As Leonardo da Vinci pointed out in his notebooks, “The black races of Ethiopia are not products of the sun, for if in Scythia a black man makes a child to a black woman, the offspring is black; but if a black man inseminates a white woman, the offspring is gray. Proof that the race of the mother has as much power over the fetus as that of the father.” Besides skin color, how do you sum up what we know — or appear to know — about heritable racial differences in traits such as intelligence, creativity, or even the rate of maturation of male voices?

Davide Piffer: As I said, creativity has been neglected by geneticists and many psychologists, so unfortunately we know next to nothing about race or individual genetic differences in creativity. With regards to intelligence, there is a growing consensus, thanks to the work of myself and colleagues, that racial differences have a genetic basis. This comes from different lines of evidence, using the most recent methods of population genetics: admixture analysis and polygenic scores.

As for voice maturation, Philippe Rusthon proposed the theory that the Mongoloids are the most K-evolved and the Negroids are the least K-evolved, while the Caucasoids fall between the two, although closer to the Mongoloids. This theory is supported by a large amount of data. I made a new contribution to Rushton’s theory by presenting data on race differences in the age at which the voice breaks in boys. The prediction from Rushton’s theory and the hypothesis to be tested is that the voice should break at a younger age in Negroids than in Caucasoids. The hypothesis was successfully corroborated.

Grégoire Canlorbe: Your most conclusive investigations in behavioral genetics deal with the connection between sexual selection and sleep patterns — or that between sexual selection and both sex- and country-level differences in performance on tests of fluid intelligence. Could you tell us more about it?

Davide Piffer: I was the first to investigate the relationship between sleep patterns and sexual behavior among humans as part of my MSc’s dissertation at Durham University. My original work was later replicated by a researcher from Sri Lanka and a group from Germany. Sleep is well known to affect mating behavior in many animal species, and some heritable differences in chronotype also predict mating success among men, both in Western and non-Western societies.

The effect seems to go above and beyond personality (extraversion) and the propensity to engage in social activities at night. Being a night owl is associated with going out at night, thus increasing the chances of meeting a member of the other sex, but also with testosterone levels. We still don’t know if it is also perceived as an attractive feature in males, but it has significant sex differences, with males across different societies being more likely to be night owls than females, so it is a candidate target for sexual selection.

Some years ago, I published a paper in which I proposed an explanation of the paradox that more developed countries, with higher equality of the sexes, had a higher sex inequality in IQ scores. That is, in more developed countries, men are smarter than women, but this sex difference is much smaller or absent in less developed countries. For example, in Muslim countries, women have higher ability than men, so we could call this the Muslim paradox. This is the opposite of what one would expect from a purely environmental perspective, because women get more education in sex-equal countries. What I found was that smarter countries had higher sex differences independent of GDP or equality of the sexes.

A potential mechanism that I did not mention in the paper but that occurred to me later is that in industrialized countries, dysgenic fertility is mainly driven by highly educated females having fewer children and later in life, whereas male intelligence is not generally related to reproductive success. This sex disparity in dysgenic fertility would cause women to decline in intelligence relative to men.

This may not seem obvious. Because both men and women contribute to the genotypes of children, the fact that high-IQ women have few children should depress the IQ of both sexes, not that of women only. The question is more subtle, however, because many complex traits have sex-specific mechanism of expression, so that the same allele has different effects in males and females, even if the genes are not located on the sex chromosomes.

Grégoire Canlorbe: You devised a methodology to detect polygenic selection, a mechanism that acts on many genetic variants simultaneously. You have done this in particular with educational attainment. How do you summarize your approach?

Davide Piffer: My approach moves away from classical methods that focused on a single gene, because most traits are polygenic — the result of contributions of many alleles, or gene variants — and the effect of each is so diluted that you need to study many genetic variants in order to detect a pattern of selection. The frequency of a single allele is mainly affected by genetic drift unless that allele has a very strong effect on a trait such as, for example, sickle-cell anemia. Natural selection also affects population frequencies of alleles, but the effect on a single allele is typically so small that it can go unnoticed. However, when you simultaneously look at hundreds or thousands of alleles, you start seeing a pattern because the effects of random drift on each allele cancel each another out, and what is left is the directional effect of natural selection, which acts more strongly on some populations than on others, according to the environmental conditions or the sexual dynamics across the millennia.

Of course, these alleles are not picked at random from the genome. They come from studies called GWAS (“genome-wide association study”) which explore the correlation between millions of genetic variants or SNPs (single-nucleotide polymorphisms) and some phenotype or trait, such as years of education or height, using very large samples (from 100,000 to over a million individuals). These studies then find the SNPs with the strongest phenotypic effect, that is those that can increase an individual’s IQ by half an IQ point or increase height by as much as a centimeter.

The average frequency of alleles weighted by their effect on the phenotype (e.g. IQ) is called the polygenic score. Polygenic scores can be used to predict how well individuals do in school, or their height, or risk for cardiovascular disease. These scores are based on direct biological evaluation of a subject’s genome, so they are unaffected by culture, family differences, or any other factor that is commonly claimed to invalidate traditional IQ testing. I calculated polygenic scores for populations by averaging the scores of samples from different ethnic groups available on public databases. In the case of educational attainment and intelligence, I used between 2,400 and 3,500 of the most significant SNPs to compute polygenic scores.

What I found was that these population-level polygenic scores had a very high correlation of r=0.9 with scores on standardized intelligence tests. Ashkenazi Jews have the highest polygenic scores, followed by East Asians, then Europeans, South Asians, Native Americans and Blacks. A correlation this high would occur at odds of only one in 46,000 if the SNPs had been chosen at random. Furthermore, the SNPs associated with the greatest intelligence differences between populations were also associated with the greatest differences between individuals within a single population. This supports the view that both kinds of difference were the result of selection.

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Grégoire Canlorbe: You made the claim that the north-south difference in Italy in fluid intelligence should be understood in terms of genetic differences between the populations. What data corroborate that hypothesis?

Davide Piffer: Historically, literacy levels and economic prosperity have been higher in the north than in the south. Data collected by Richard Lynn have shown that these differences are reflected in scores on tests of intelligence and scholastic aptitude (PISA and INVALSI).

There are strong genetic differences within Italy, recently corroborated by an in-depth study (Raveane at al., 2019). These differences were mostly established in pre-Roman times: Bronze-Age migrations from the Eastern European steppes (Indo-Aryans) in the north and West Asians from the Caucasus in the south. The Latin people who founded ancient Rome belonged to the Indo-European/Aryan group, along with other Italic tribes, such as the Veneti who later founded Venice.

Later migrations strengthened the pre-existing differences. In the first millennium BC, groups of Celtic people (originating from Indo-Aryans, like the native northern Italians such as the Veneti and the Ligurians) settled in the north and mixed with their ethnic cousins (that is why the Roman name for northern Italy was Gallia Cisalpina), and the Greeks heavily colonized the South (Magna Graecia) in the 1st millennium BC. In the Middle Ages, Germanic people invaded Italy and mostly added to the genetic pool of northern Italy (mostly Lombards and Gothic peoples) with some pockets in central and southern Italy (the Lombards in the Ducato di Benevento and the Normans in Palermo), whereas the south was conquered by Arabs.