This is clearly speculation? Why not "This is clearly a speculation"?

It seems to me the article is necessary.

Context:

We can also now begin to explain the origins of a tiny
fraction of the more mechanical differences between us and
our closest relatives, some of which may play crucial roles in
our humanness. In one example, a gene for a jaw muscle pro-
tein (MYH16) appears to have mutated into a pseudogene in
humans. It continues to play a significant role in the develop-
ment and strength of the jaw muscles in other primates. It is
just conceivable that the inactivation of this gene led to a re-duction in the mass of the human jaw muscle. Most apes have relatively larger and stronger jaws than we do. Human and ape skulls must, among other things, serve as an anchor for these jaw muscles. It is possible that the development of weaker jaws paradoxically allowed our skulls to expand upward, and accommodate our larger brains. This is clearly speculation, of course, and other genetic changes would be
necessary to account for the much larger brain cortex that