In tracing out the folds of life it is often said that the higher forms have originated from lower forms. So the ancestor of mammals must be some lower vertebrate forms. The chances for the fishes and the birds are out of question, because one is too low and the other is highly specialised.
Ancestry from them is in no case possible. Now remains the possibilities for the amphibia and the reptiles to hold the respected position of mammalian ancestor.
The amphibian ancestry of mammals was emphasised by T. H. Huxley in the year 1880. He advocated that mammals arose from amphibia. He assumed that Hypotheria is a sub- mammalian evolutionary stage, which arose from amphibia and holds the key of mammalian evolution.
In support of his contention, Huxley put forward some arguments. First, amphibia as well as mammals possess double occipital condyles.
Another argument in this context is that in mammals only the left aortic arch is present but in amphibians and reptiles both the arches are present. Of the two arches, the left one is weakly developed in reptiles which indirectly supports amphibian ancestry of mammals.
But this theory was subjected to vigorous criticism.
The points of resemblance are not rigid enough to support Huxley’s assumption excepting the occurrence of double occipital condyles. The occipital condyles in amphibians and mammals are’ not strictly homologous structures but differ in development.
In amphibians the occipital condyles are the derivatives of the exoccipitals, but in mammals the source is the basioccipital. This assumption is further weakened by the lack of any intermediate forms and palaeontological data.
Huxley’s view of mammalian origin through Hypotheria is untenable today and does not offer any tangible evolutionary significance. In spite of that this view is still kept as a piece of historical document in the discussion of mammalian ancestry.
The very concept of mammalian ancestry directly from amphibia without the intervention of reptiles is most unscientific and absurd.
The reptilian ancestry of mammals is full of evolutionary significance and gives a vivid indication of biological transformation towards the mammalian organisation. The reptilian ancestry of mammal can be established from three angles,
- Evidences furnished by some living mammalian groups,
- From serological tests and
- From palaeontology.