So I was thinking about making a post about feather evolution - since we already have a post about hair evolution and feathered dinosaurs are, in my opinion, some of the coolest things out there - when I came across an article entitled Mammary glands and feathers. WTF. This deserves a post.
|Archaeopteryx. Just because.|
So what exactly do mammary glands and feathers have in common? It seems quite a lot.
First of all, the basic structures between mammary glands and feathers are quite similar. Both are skin appendages that originate from epithelium and mesenchyme germ layers. These skin appendages are both adaptive. For birds, feathers serve as insulators, and help with communication and flight. For mammals, mammary glands provide essential nutrients for young when foraging would be costly and dangerous.
Feathers likely evolved in steps. Although the exact nature of these steps is complex and debated, Richard Prum developed the first tentative theory of feather evolution you can read about here and here. What seems certain, however, is that many dinosaurs posses intermediate feather forms. These "primitive" feathers probably served as insulation for emerging endotherms, and only later acquired their role in flight.
|The steps in the evolution of feathers|
Mammary glands, like feathers, evolved in a stepwise fashion. At first, these glands were probably all over the skin and may have helped moisten eggs. These glands would then have clustered to a single region, and been modified to contain nutrients beneficial to the young. To increase the efficiency of milk delivery, the areola and the nipple would have then formed. Finally, the mammary gland would have become the object of sexual selection, as it is today in humans.
Finally, feathers and mammary glands also contain stem cells, and their growths are regulated by sex hormones (note this is only true for some feathers). They also both eventually acquired essential roles in sexual selection.
Although these connections may seem arbitrary, they illustrate a few important points about evolution. Ectodermal organs aren't usually considered to be essential for an organism to survive. This means modification of skin appendages are less likely to be deleterious, and a variety of adaptive skin appendages, like mammary glands and feathers, can evolve. Once these appendages have initially been developed, they integrate with the rest of the body - for example, feathers work with muscles and nerves to control flight. The authors suggest that skin appendages can be good model organs for exploring Evo-Devo, precisely because they are so connected to the rest of the body and are so accessible.
All in all, it's an interesting parallel.
Widelitz R., Veltmaat J., Mayer J.A., Foley J., Chyong C.M (2007). "Mammary glands and feathers: Comparing two skin appendages which help define novel classes during vertebrate evolution." Seminars in Cell & Developmental Biology. 18: 255-266
Prum R. (1999). "Development and Evolutionary Origin of Feathers." Journal of Experimental Zoology. 285: 291-308
Chuong CM, Chodankar R, Widelitz R, Jiang, T. "Evo-Devo of feathers and scales: building complex epithelial appendages" Current Opinion in Genetics & Development 10:449–456