What did you think of the reading?
Once again, through his use of descriptive anecdotes and insightful ventures, Neil Shubin enlightens the reader as to the surprising connections between our development and evolution as human beings, as compared to seemingly unrelated organisms. As established by the first part of the book, the next few chapters expounded upon the prevalent theme of tracing the same organ in distant species. As the focus switches from fossil excavation to DNA sequencing to features of the head, the smooth transitions and underlying theme enforce the take-home lesson of human evolution through primitive organisms (as well as the presence of a common ancestor and proof of evolution). All three of these resound with biological significance that the author addresses and highlights in a clear and captivating manner. As stated before, the pictures and their accompanying captions serve well to convey these messages as well.
What did you learn from the reading?
The chapter based on genes and DNA sequencing in humans effectively utilized comparative embryology in order to liken the developmental changes of humans to simpler organisms, such as flies and chickens. Referring to embryo limb development, similarities among fly, chicken, and shark eggs were addressed. Interestingly, observations in the anatomy of certain (simpler) organisms led to breakthroughs in other (complex) species. Take for example the hedgehog gene. The Zone of Polarizing Activity, or ZPA (which can be observed in the human hand) is analogous to the hedgehog gene, which can be found in bodily segmentation of flies. A similar version of this gene was also found in chickens; the new version was coined the Sonic hedgehog gene. Experiments in the embryological development of such organisms answered my previous question regarding limb development via coding versus developments via environmental conditions. Depending on where the ZPA is located, different results in limb development is noticed.
The next stretch of the book focused more on the biological processes of making organs. Despite the wide array of organs and their different characteristics in unrelated species, the organs are formed by different versions of the same process; it isn’t just a different code for each body of each species. Starting the chapter on teeth, I discovered the evolution of teeth based upon information expounded from fossil records. Since the tooth is the hardest part of the body (due to elevated levels of hydroxyapatite present in this organ), paleontologists have access to a huge amount of information based on these remains. In the same way the author found the Tiktaalik, which represented fish’s transition from water to land, the tritheledont represents the transition from a reptile to a mammal. This is evident from the presence of mammalian tooth-to-tooth occlusion in a primitive reptile. This chapter also encompassed more tidbits about fieldwork, and paleontology- the author’s main profession.
What questions do you still have?
As a reader, I still have questions regarding the motivation behind the monumental changes of animals as time passed from prehistoric eras up to the present. For example, the author mentioned the fish’s transition from water to land as a response to predation; this same reason was also utilized for the motivation behind transitioning from reptilian to mammalian jaw features. However, is this the only reason for this transition? Did drastic changes in habitat play a role in the evolution of animals? Food source? Location on Earth? Also, if every species’ organ development was a variation on a theme, then what barrier keeps these variations separated among organisms? In other words, what keeps a reptile’s development like a reptile and not like a mammal’s (and vice versa)? I understand the “why” behind that question; obviously, nature must maintain diversity to remain healthy, but what barrier ensures this?