One of the hallmarks of human evolution, aside from our bipedalism and extraordinarily large brains, are our forelimbs… especially the famed prehensile thumb. Our forelimbs, or arms, are extremely flexible compared to a quadruped. For example, because of the shallowness of the ball and socket joint that connects our humerus to our scapula, our arms can rotate 180 degrees. A special thank you to our arboreal tree swinging ancestors. But our arms aren’t that different from other organisms. Actually, if you’ve spent any time reading your basic biology textbook, you probably have come across a common illustration which compares the human forelimb to forelimbs of other organisms, such as the bat, whale, etc.
In these illustrations, you are supposed to see how the bones that make up a bat’s wing are structurally analogous to both human hands and seal flippers, due to the common descent of these structures from an ancestor that also had five digits at the end of each forelimb. The bones of the bat wing are proportionately different from a human’s arm, but they still share the major components such as the humerus, radius, and ulna. These illustrations are ultimately meant to document that a lot of homology exists in the basic vertebrae body plan and establish the fact that all vertebrates share a common evolutionary ancestor.
Based off of papers such as this 2006 Science publication, we’ve come to understand that the structural homology is due to the activation and expression of several critical genes that regulate development. One of them is called sonic hedgehog or SHH, named after a favorite video game of the researcher who discovered the gene. When there are mutations in SHH, limbs do not develop normally, as illustrated in the collection of skeletons from the various mutant stocks to the right. The forelimbs of the top represent normal development with the presence of SHH. In the absence of SHH signaling, forelimbs are mutated as seen in the bottom.
Comparing the SHH sequence between organisms have shown that it is a highly conserved gene, found with little difference in species as diverse as arthropods and mammals. In 2006 we also saw another paper that figured out the molecular evolution of SHH is relatively accelerated in primates, when compared to other mammals. And SHH is even more accelerated in the human lineage. Since SHH is a gene expressed during and for development, such findings implicate SHH as a potential contributor to the evolution of primate and human specific morphological traits.
Anyways, where I’m trying to go with this is that we see a conservation in limb development, both genetically and phenotypically. Neil Shubin, a professor of anatomy at Chicago University also investigated this phenomenon and published his findings in his book, “Your Inner Fish: A Journey into the 3.5 billion-year History of the Human Body“, which explores the links between humans and their most ancient forebears.
He’s analyzed the tiktaalik fossil, which is supposed to represent a transitional species of fish to amphibian. By the way, the tiktaalik discovery was announced also in 2006 by Shubin and colleagues. In his comparisons, he sees that ours wrists and unique opposable thumb, even the shape of our skulls, can be traced to origins in the tiktaalik. Shubin also found out that the tiktaalik fossil displayed similarities to the human shoulder, elbow, and forearm.
“When we study the structure of these joints to assess how one bone moves against another, we see that tiktaalik was specialized for a rather extraordinary function – it was capable of doing push-ups,” writes Shubin.
Separately, Shubin has found that modern-day fish carry genes allowing for the growth of wrists, hands and fingers. These are now “switched off” so the digits never develop in the fish.
Such findings cast doubt on the assumption that hands are a more recent evolutionary step than fins. Instead, fins may have developed as an improvement on hands.
The research also supports the argument that the majority of the human genome developed 500m years ago and is shared with most living creatures.
One of the factors that makes living forms different is the ability to switch off certain genes while retaining them in the genome.
An alternative approach is to adapt similar genes to different purposes. Some of the genes involved in the evolution of human vision and hearing play an active but very different role in the metabolism of jellyfish.
I feel that excerpt was awfully simplistic, but I want to bring to attention what I’ve bolded. The second statement, the one that says the majority of the human genome developed 500,000,000 years ago is an extraordinary but not nearly as controversial statement as the first one. We know that genes like SHH and the HOX genes are conserved… and being eukaryotes, many of the genes that encode for basic functions of the cell are conserved.
But where Shubin is paraphrased, saying fish have the developmental genes that pattern for wrists and fingers but don’t express them, is ballsy. Furthermore, saying limb patterns that makes up a hand developed before fins is even more of a contentious statement. It is reminiscent the curve ball Aaron Filler’s threw us in his human ancestor for the apes hypothesis, because the current understanding is based upon fish with fins gave rose to amphibians that have more hand-like forelimbs than their fishy ancestors. I’d like to know what genes Shubin has identified as inactivated in fish for the development of hands, wrists, fingers in fish are. But unfortunately the source article doesn’t mention them. I guess I gotta buy and read the book.