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Thursday, April 19, 2018

Why Has Human Physical Evolution Stopped?

Perhaps my headline is a bit misleading, but researchers at the University of Tokyo have found that vertebrates may have conserved their basic anatomical architecture, or bodyplan, for over 500 million years by reusing their genes.

Simply put, reusing genes constrains diversification. 

Some background (ha-ha): Vertebrates are a group of animals that have vertebrae in their back, which is to say vertebrae are the individual, interlocking bones that form the spinal column. In humans, we have 33.

The vertebrates emerged more than 500 million years ago and diversified into a variety of species comprising a multitude of shapes, including humans and other mammals, birds, reptiles, amphibians, and fishes.

After the diversification so many years ago, why haven’t the basic architecture of the vertebrates changed very much.

All vertebrates have retained their basic anatomical architecture through evolution spanning several hundred million years, and scientists had not yet pinned down why this is so, though the University of Tokyo thinks it might have an answer.

While previous studies had attributed the lack of anatomical diversification to the embryonic phase when vertebrates' basic architecture develops, recent studies highlighting the evolutionary conservation of this phase in what is called the developmental hourglass model. Nonetheless, why this embryonic phase was conserved through such a long evolutionary time scale remained unresolved.

The international collaborative group, EXPANDE consortium, led by associate professor Irie Naoki (surname first) of the Graduate School of Science at the University of Tokyo tackled this problem by comparing the gene expression profiles during development of the embryos of eight species in a larger grouping of animals, called chordates, which comprises lancelets and tunicates, in addition to vertebrates.

Chordates… I had to look this up on Wikipedia, is an animal that possess a notochord (a cartilaginous skeletal rod supporting the body in all embryonic and some adult chordate animals), a hollow dorsal nerve cord, pharyngeal slits (repeated filter-feeding openings that appear along the pharynx caudal to the mouth), an endostyle (an organ which assists lower-chordates in filter-feeding, secreting mucus which utilizes cilia to coat itself), and a post-anal tail (an extension of the body that runs past the anal opening. In some species, like humans, this feature is only present during the embryonic stage), for at least some period of their life cycle. Chordates are deuterostomes, as during the embryo development stage the anus forms before the mouth.

Clearer now? Sorta, but no, me neither.

Chordates can include creatures such as mammals, fish, birds, reptiles, and amphibians (all of which are vertebrates), as well as sea squirts (tunicates) and lancelets (celpalochordates).

Okaaaaaay… still not getting it.

What the fug is a sea squirt? This is a type of sea squirt:
Komodo National Park Gold-mouth sea squirt (Polycarpa aurata), by Nhobgood Nick Hobgood in 2006.
The sea squirt above, while looking like a human heart thrown into the water for a week, is actually quite pretty in its coloration. Still, that must have been one brave diver who first picked up this type of sea squirt... and even braver is the person who looked at that and said " I wonder if I can eat that?"   
And yes, humans eat them. Maybe not this particular variety, but many others, such as these live sea squirts known as a sea pineapple, found for sale at a market, Busan, South Korea:

I'm pretty sure these sea pineapples don't taste like "land" pineapples, but I've been wrong before. I'm not wrong this time. Photo by ProjectManhattan
By the way, another type of tunicate is a similar-looking creature called “sea pork” which luckily for it does not taste like real pork.

Examples of sea pork on the beaches of Hilton Head, South Carolina, US: photo from 
A lancelet is 32 species of fish-like critters that tend to bury themselves in the sandy waters, exposing only their head.

This is a lancelet:
Lancelet feeding on plankton. Photo by Colin Gray

Chordates include: humans, alligators, pandas, crows, sharks, salamanders, and much, much more.

The University of Tokyo researchers identified and compared the genetic data on these species, and found that most of the genes acting around the developmental phase, which shapes the vertebrates' basic architecture, were pleiotropic genes—genes producing more than one effect—that were reused repeatedly.

Notably, the researchers found a strong correlation between the ratio of such repeatedly recruited genes and evolutionary constraints.

One plausible scenario is that the embryonic phase, in which the vertebrates' basic architecture develops, is enriched with repeatedly reused genes, which constrain evolutionary diversification. In other words, it helps prevent mutations.

Of course, mutations are what got us to where we are, and what we are, today.

You all know the old chicken and the egg scenario and conundrum about which came first… well, obviously eggs were around long before there were chickens. And, whatever laid the egg that would become the first chicken, wasn’t a chicken… it was something that had a mutatable gene within it. 

Since gene recruitment is commonly observed during evolution in a variety of organisms, the study's findings promise to provide a basis for better understanding what kind of trait is likely or not so likely to evolve.

"In the beginning, I wasn't expecting to find what we discovered. In fact, I was reluctant to do the analysis that led to our main finding because I thought it would be meaningless," says Irie. "Considering our knowledge of its contribution to the evolution of many new traits, gene recruitment could be a double-edged sword in that its recurrence constrains evolution. Our findings raised a lot of additional questions in my mind for further inquiry."

Thank you to the folks at the University of Tokyo for continuing to send me their press released on their scientific research gains!

Andrew Joseph

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