KERATIN AND MOLECULAR BIOLOGY 



25 



activity in the cells forming the protein. Thus the appearance of a new 

 type of protein, such as feather keratin, would seem to imply a mutation, 

 which in this case must have occurred in the reptilian stem-line after the 

 mammal-like reptiles had branched off, but before the birds separated 

 from the main stream (Heilmann, 1926) (Fig. 13). An appropriate new 

 protein appearing in the epidermal cells would presumably be subjected 

 to the same processes of keratinization as its predecessor. In the instance 

 of feather keratin, its utilization by reptiles, the earlier forms, would seem 



REPTILES, BIRDS 



-<?}- Collaqen 

 -@- Chitin 

 -0- Keratin 



Fig. 13. Distribution of collagen, chitin and keratin shown on a con- 

 ventional phylogenetic tree. 



to be a simple replacement of the commoner a-type keratin conferring no 

 advantage obvious to us. In birds the situation is very different. The 

 /3-type molecule, as will be shown in Chapter 5, is inextensible and inflexible 

 in contrast to the (extensible) a-type molecule (also the basis of muscle) 

 and forms an admirable structural foundation for the economical con- 

 struction of feathers which must be both light and stiff. The successful 

 invasion of the air is thus partly based on the exploitation of this new type 

 of epidermal keratin. 



With a sufficient knowledge of the distribution of types of macro- 

 molecules among existing animals and plants, combined with a knowledge 

 of the actual evolutionary descent of organisms, it would seem possible 



