Conduta e Evolução dos Cetáceos

Exaptação e conduta - seguidos por câmbios epigeneticos desengatilhados pelo novo nicho e deriva estrutal de estruturas livres para se transformarem - são dois agentes poderosos no processo evolutivo. Certa vez li um caso semelhante discutido por Vargas e Rubilar em relação a especilização das aves para o ovo. Encontro agora um outro exemplo não menos atrativo - a evolução inicial da linhagem dos cetáceos:

Nature 450, 1190-1194 (20 December 2007)

Whales originated from aquatic artiodactyls in the Eocene epoch of India

J. G. M. Thewissen1, Lisa Noelle Cooper1,2, Mark T. Clementz3, Sunil Bajpai4 & B. N. Tiwari



"The modern artiodactyl morphologically most similar to Indohyus is probably the African mousedeer Hyemoschus aquaticus. Hyemoschus lives near streams and feeds on land, but flees into the water when danger occurs40. Indohyus had more pronounced aquatic specializations than Hyemoschus does, and it probably spent a considerably greater amount of time in the water either for protection or when feeding. As indicated by the evidence from stable isotopes, Indohyus spent most of its time in the water and either came onshore to feed on vegetation (as the modern Hippopotamus does) or foraged on invertebrates or aquatic vegetation in the same way that the modern muskrat Ondatra does.

Raoellids are the sister group to cetaceans, and this implies that aquatic habitats originated before the Order Cetacea. The great evolutionary change that occurred at the origin of cetaceans is thus not the adoption of an aquatic lifestyle. Here we propose that dietary change was the event that defined cetacean origins; this is consistent with the cranial and dental synapomorphies identified. Molars of Indohyus are markedly different from those of pakicetids, and it is widely assumed that pakicetids ate aquatic prey18, 23.

Our working hypothesis for the origin of whales is that raoellid ancestors, although herbivores or omnivores on land, took to fresh water in times of danger. Aquatic habits were increased in Indohyus (as suggested by osteosclerosis and oxygen isotopes), although it did not necessarily have an aquatic diet (as suggested by carbon isotopes). Cetaceans originated from an Indohyus-like ancestor and switched to a diet of aquatic prey. Significant changes in the morphology of the teeth, the oral skeleton and the sense organs made cetaceans different from their ancestors and unique among mammals."



Poderia ser diferente? Os pobres cetaceos, a partir de uma nova conduta, se tornaram aprisionados en el mar, como Glaucus: "According to Ovid, Glaucus began life as a mortal fisherman living in the Boeotian city of Anthedon. He discovered by accident a magical herb which could bring the fish he caught back to life, and decided to try eating it. The herb made him immortal, but also caused him to grow fins instead of arms and a fish's tail instead of legs forcing him to dwell forever in the sea".

By the way, hoje morreu o grande compositor brasileiro Dorival Caymmi, que, junto a Jorge Amado, escreveu a linda canção "Doce é morrer no mar".