Geographic Location on the Origin of Angiosperms

The below mentioned article provides a short notes on Geographic Location of the Origin of Angiosperms.

The geographic origin of the angiosperms has been under considerable debate. Just as the fossil evidence is insufficient to decide the time, as well as the nature of ancestors of angiosperms, it is also inadequate to settle the problem of the place of origin.

There are three divergent views regarding the place of the origin of angiosperms, which are under mentioned:

(a) The Hypothesis of Arctic (or Antarctic) Origin:

The dominant view regarding the origin of angiosperms during the late nineteenth and early twentieth centuries, had been that the angiosperms originated high in the northern latitudes and even in the polar-regions.

This hypothesis holds that angiosperms originated in the polar region of Holarctic and from the polar Arctic and Antarctic regions, the angiosperms spread in successive waves across the whole earth. It provides an easy explanation for the present distribution of many genera in widely separated continents, chiefly the northern America and the eastern Asia.

This view has however been disputed for a more tropical “centre of origin” (between 45° N to 45°S), as at the beginning of the Cretaceous, angiosperms were found only between 45° N and 45°S latitudes and not beyond these, from which they migrated towards the polar regions.

The fossil evidence suggests that it was not until the Late Cretaceous that angiosperms established in the Arctic, as angiosperm fossils from the Early Cretaceous have been found only at low latitudes.

According to Hickey et al.  the Arctic may have been an important site for some level of floral evolution in the Late Cretaceous. It is only at the end of the early Cretaceous that the angiosperms reached higher latitudes, eventually replacing the relict Jurassic-flora at higher latitudes towards the end of the Mesozoic.

Thus, Takhtajan is of the opinion that “the hypothesis of Arctic (or Antarctic) origin for angiosperms” must be firmly rejected.

(b) Theory of the South-East Asian Centre of Origin:

The angiosperm centre of origin has been predicted to be in south-eastern Asia. Hallier proposed that the angiosperms originated first in the basin of the Pacific Ocean, at places such as New Zealand, New Caledonia, Andes and the Hawaiian Islands.

Takhtajan has suggested that the ‘cradle of the angiosperms’ occurs somewhere between Assam and Fiji (including Burma, Thailand, Indo-China, and Malaysia), due to the high abundance of primitive angiosperm families (e.g. Amborellaceae, Austrobaileyaceae, Calycanthaceae, Degeneriaceae, Eupomatiaceae, Gomortegaceae, Himantendraceae, Lactoridaceae, Magnoliaceae, Trochodendraceae, Tetracentraceae, Winteraceae, etc.) found in that region of the Pacific basin.

However, this does not mean that all primitive angiosperms are concentrated in the South-Eastern Asia. They occur outside this region as well.

Takhtajan therefore considers south-east Asia to be the most likely region where angiosperms originated, and descendants of the first angiosperms may have dispersed far from their point of origin.

The lack of fossil evidence is the serious drawback of this theory. The present acceptance of the theory is largely based on the study of distribution and comparative morphology of presumably primitive angiosperms.

(c) “Species Pump” Hypothesis:

According to Stebbins, the tropical rain forests with equable climate have most probably served as the “museums” rather than “cradle” of angiosperms i.e., they are not the communities within which angiosperms originated and differentiated, but they have preserved the diversified plant groups because of low rate of extinction.

He proposed that active and rapid speciation, leading to the origin of diversified groups of angiosperms is likely to occur in marginal or emotional regions of tropical forests, due to increasing drought.

The museum hypothesis is supported by the fact that tropical forests do not contain any transitional forms or when present they are less abundant. However, they contain a complete spectrum of families ranging from the most primitive to the most advanced forms.

According to the “species pump” hypothesis developed by Valentine, the newer populations produced in the marginal areas are “pumped” into the stable communities of tropical forests from time to time.

Thus, angiosperms, in all probabilities have developed in semi-arid situations, marked by seasonal drought and a short season favourable for the formation of flowers and seeds.

The regions in which climatic factors have greatly promoted the diversification of modern flora are the Ethiopian highlands, the Cape region of South Africa, parts of the Indian Peninsula, the coast of Ecuador, south-central Mexico, northern Venezuela, and the northern provinces of Argentina.