Thursday, December 14, 2017
EVOLUTION: The Eureka Moment
The “eureka!” moment, when I saw the Pogonophora as the significant link of the two main divisions of higher animals, can be credited to the stimulation of reading Gans and Northcutt, 1983.
Gans, Carl, and R. Glenn Northcutt. 1983. Neural crest and the origin of vertebrates: a new head. Science, 220:268-274.
They placed the pogonophorans in line with the vertebrates based on development. I was sure the evidence was overwhelming that pogonophorans were close to, or one of, the annelids. But I also realized most scientists are honorable and truthful in their work and deserve to be taken seriously. But how could Gans & Northcutt be right when the overwhelming evidence indicated pogonophorans were close to annelids and other protostomes? Somehow, in an instant, I realized it could be true if pogonophorans were a connecting link. A deluge of such evidence came to mind. And, as I followed new, as well as some older, molecular and other evidence the connection became well supported.
Engemann, Joseph G. 1968. Pogonophora: the oldest living animals? Pap.
Acad. Sci., Arts, and Letters,
Engemann, J. G. 1983. Coelomate animals are monophyletic. American Zoologist, 23(4):1008. Abstract # 753. The Pogonophora have characteristics of both protostomes and deuterostomes and provide support for the annelid theory of origin of deuterostomes.
Understanding the extreme age of individual pogonophorans, suggested in the 1968 report above, was a result of preparing a new section on pogonophorans for the 1968 edition of Hegner and Engemann’s Invertebrate Zoology text. It was reprinted in chapter 14 of the 1981 edition (Engemann and Hegner) which discussed the evidence making it very likely deep-sea animals typically have very extended lives and low respiratory rates. My 1983 abstract noted above was reported shortly after Gans and Northcutt triggered my conclusion with their evidence.
A full report of the paper was submitted to Nature. The reviewers did not reject the paper but the editor decided not to publish it because it was not of wide enough interest. I had given it a title suggesting pogonophorans were the protostome-deuterostome link. He was not moved by my suggestion that a catchier title would have been “my ancestors were worms”.
Of course, there is a whole sequence of organisms from protozoans through sponges, jellyfish, flatworms, fish, amphibians, reptiles, insectivores, primates and closer relatives in our direct lineage. But we don’t have direct ancestry through either nematodes, mollusks, arthropods, echinoderms, or many other groups. If people squirm to think some ape-like primate was in our evolutionary ancestry, how much more appropriate to squirm for a worm.
What about extreme longevity of pogonophorans?
It helps explain their slow evolutioary rate, and thus, their close molecular relationship to diverse groups of animals.
What is so important about abyssal life of pogonophorans?
The slow pace of life at great depths, due to great pressure, low food and oxygen input to the depths, paucity of life, probable absorption of fossil nutrients from sediments, and isolation from many surface extinction factors makes them living "fossil" ancestors. [Note: really old people may live to see great, great, great grand-children]
What has pressure to do with it?
It has not been demonstrated but it is obvious that reduced diffusion based metabolism is probably the missing factor in reduced community respiration noted at great depths. I await someone making observations of reduced Brownian movement and/or diffusion of dyes at great depths. It may be a factor in extended submersion time for deep-diving whales. Water is ever so slightly compressed at great pressures- it may be the cause.
Could circulatory systems increase activity and decease longevity at great depths?
Perhaps. But whales presumably shut down some less essential portions of theirs.
What about the great difference in early embryology of the groups alleged to be connected by the pogonoporans?
That has been discussed in other posts. Also, observation of isopod development in Tasmania and Michigan gives some clues to different rates of development associated with ecological factors. Abyssal life put a species survival premium on shifting from protostome to deuterostome development.
Joseph G. Engemann Emeritus Professor of Biology, Western Michigan University, Kalamazoo, Michigan December 14, 2017