January 20, 2021 | David F. Coppedge

Any Evolution for Platypus Yet?

It’s been a long time since scientists have tried to Darwinize the platypus. Any progress?

The duck-billed platypus and the echidna are the only two members of a group of marsupial mammals called monotremes. They lay eggs instead of bearing live young. The platypus looks like a hodgepodge of unrelated animals. Its original scientific name Ornithorhynchus paradoxus, “bird-snouted paradox,” (now Ornithorhynchus anatinus) indicated the puzzled looks its 1798 discoverers wore when first looking at it. Has the mystery of platypus evolution been solved since the 2016 Aug 1 attempt?

How Earth’s oddest mammal got to be so bizarre (University of Copenhagen). Here comes the Kipling-form JSS (just-so story): “How the X got its Y.” This will be a test of the explanatory power of Darwinian evolution. Anticipating some good scientific work, let us approach the story with an open mind, seriously and respectfully.

Often considered the world’s oddest mammal, Australia’s beaver-like, duck-billed platypus exhibits an array of bizarre characteristics: it lays eggs instead of giving birth to live babies, sweats milk, has venomous spurs and is even equipped with 10 sex chromosomes. Now, an international team of researchers led by University of Copenhagen has conducted a unique mapping of the platypus genome and found answers regarding the origins of a few of its stranger features.

We reduce our expectations accordingly. They will not give a full account of all the bizarre characteristics; just some of them. That is acceptable. They mapped the platypus genome, which is legitimate work. Their findings are published in Nature, they say; we’ll get back to that later.

“The complete genome has provided us with the answers to how a few of the platypus’ bizarre features emerged. At the same time, decoding the genome for platypus is important for improving our understanding of how other mammals evolved — including us humans. It holds the key as to why we and other eutheria mammals evolved to become animals that give birth to live young instead of egg-laying animals,” explains Professor Guojie Zhang of the Department of Biology.

Whether it improves understanding of human evolution remains to be seen. That appears to be an unwarranted extrapolation. First we need to know if the genome answers how platypus features “emerged” (a magic word till demonstrated). We look for demonstrations of evolution, not assertions.

Figure 1. Duck-billed Platypuses. Notice this fur-covered mammal has a duckbill, webbed duck feet and a body with short legs like a Dachshund. From Wikimedia Commons.

The platypus belongs to an ancient group of mammals — monotremes — which existed millions of years prior to the emergence of any modern-day mammal.

“Indeed, the platypus belongs to the Mammalia class. But genetically, it is a mixture of mammals, birds and reptiles. It has preserved many of its ancestors’ original features — which probably contribute to its success in adapting to the environment they live in,” says Professor Zhang.

Assertions held together by speculations is not helping. The press release discusses genes for milk, called vitellogenin genes. Chickens have three of these genes. The article begins telling a story of how humans lost all 3 of them and replaced them with casein genes, but the platypus lost two of the vitellogenin genes. This is odd, since mammals are not on the bird lineage according to Darwinians. How can they say what humans “used to have” without assuming Darwin’s tree – a circular argument?

“It informs us that milk production in all extant mammal species has been developed through the same set of genes derived from a common ancestor which lived more than 170 million years ago — alongside the early dinosaurs in the Jurassic period,” says Guojie Zhang.

They fail to show a picture of the common ancestor. What about that strange duck’s bill?

Another trait that makes the platypus so unique is that, unlike the vast majority of mammals, it is toothless. Although this monotremes’ nearest ancestors were toothed, the modern platypus is equipped with two horn plates that are used to mash food. The study reveals that the platypus lost its teeth roughly 120 million years ago, when four of the eight genes responsible for tooth development disappeared.

Echidnas lack teeth as well. If anything, this is devolution, not evolution. Losing genes is not going to get from bacteria to humans. Perhaps like blind cave fish, the monotremes get by without something that the Darwinians think it should have. Tell that to the platypus: “You need teeth!” The platypus replies, “I have no need of that hypothesis.”

The last attempt to Darwinize the platypus is to explain its 10 sex chromosomes.

Thanks to the near-complete chromosomal level genomes, researchers can now suggest that these 10 sex chromosomes in the ancestors of the monotremes were organized in a ring form which was later broken away into many small pieces of X and Y chromosomes. At the same time, the genome mapping reveals that the majority of monotreme sex chromosomes have more in common with chickens than with humans. But what it shows, is an evolutionary link between mammals and birds.

It’s hard to see how this shows an evolutionary link. Maybe the Nature paper can help elucidate the evidence to make it rise above JSS rank. The paper appears to only offer alternative scenarios about what might have happened. Without demonstrable proof, there are an infinite number of alternative scenarios that fit what some persons consider plausible.

A chromosome ring configuration has been reported in plants, but not in any animal species. Alternatively, the ancestral ring structure might have evolved after the emergence of the proto-X1–Y5 pair by translocations that involve other autosomes, so that sexually antagonistic alleles could be linked to the sex-determining genes.

Scientists should know not to build an explanation on single instance.

Use of “might have evolved” and “emergence” keeps this explanation down in the bottom shelf where the fiction books are. The paper uses “selection” three times, only as possibilities for what might have happened. It refers to “ancestor” and “ancestral” 45 times, but there are no drawings of the ancestor or fossils of it. There is no mention of positive selection. There is no origin of novelty. “Evolution” appears 122 times in the paper, but where, exactly, is the evolution?

Complete and accurate reference genomes and annotations are critical for evolutionary and functional analyses. It remains a challenge to produce a highly accurate chromosome-level assembly, particularly for differentiated sex chromosomes. We have produced a high-quality platypus genome using a combination of single-molecule sequencing technology and multiple sources of physical mapping methods to assign most of the sequences to a chromosome-scale assembly. This permits better-resolved analyses of the origin and diversification of the complex sex chromosome system that evolved specifically in monotremes. We delineate ancient and lineage-specific changes in the sensory system, haemoglobin degradation and reproduction that represent some of the most fascinating biology of platypus and echidna.

The paper fails to mention the origin of the truly novel traits in the platypus: the duck bill with its electrical sensors, the poison spur, the fluorescent fur, the webbed feet, nor the different lifestyles of the only two monotreme species. No transitional form is described.

The main thing they brag about is the story for how the sex chromosomes got divided into 10 of them. That hypothesis, however, involves only rearrangement of existing information. Their phylogenetic analysis appears to only compare vastly different organisms, like frogs, chickens and humans, with the opossum (a marsupial) the closest. No pre-platypus is shown looking anything like the modern platypus. The paper ends with a promissory note for futureware to fill in all the blanks:

The new genomes of both species will enable further insights into therian innovations and the biology and evolution of these extraordinary egg-laying mammals.

Without the beginning assumption of evolution, this paper would have no limbs to hang the observations on.

Verdict: just-so story.



(Visited 829 times, 1 visits today)

Leave a Reply