The fact that the Cetaceans are splendid and efficient marine creatures does not mean that always they have been it. Their evolutionary history is as interesting as uncertain; the fossil rests are in fact insufficient and fragmentary.

The supporters of the theories that try to answer to the question if the evolution is the result of accidental genetic modifications in the individual patrimony, such to allow to the same individual, a better adaptation to the ambient in which lives (C. Darwin ), or if it is the same ambient to influence directly genetic modifications (J.B. Lamarck ), they interrogate themselves on which have been the evolutionary mechanisms that have carried originally land animals, to radically modify therefore their life style.
Approximately 65 million years ago (Paleocene) the uncontested dominion of the Dinosaur on the earth finished and those that until then had been small, timid animals, began their rise to true dominating of the planet, were be a matter of the Mammals, ready to occupy every ecological niche left free, assuming therefore the shapes and dimensions more disparate .
Ten million years after, Mammals had already created many species, but in particular only one interests us, draft of the Mesonychide, a large land predator of aspect and dimensions variable (along in average 1.5 m) supplied of one skull a lot developed and large teeth, carnivore who did eat probably fishes and carrions.
The aspect of the emerged lands on which the Mammals were developed was much various from the one of our days, the so-called drift of the continents was, therefore like now, modifying the disposition of seas and lands. In the area of the current Mediterranean Sea, between Europe and Africa, existed, 55 million years ago (the Eocene), the so-called Sea of Tethys, one immense marshy area and brackish from with few deep sounding-depth and mild climate
It will be just the Mesonychide to giving origin to the first cetacean said Archeoceti (from Greek archýs "the ancient"and from the Latin cetus"cetaceo") and to the Condylarthra an ungulate mammalian progenitor of the current ungulates (ex. horse, camel, ox etc).
The study of the Archeoceti has allowed to specify several species grown in the following 30 million years, the Ambuloceti, the Protoceti, the Pakiceti, the Dorudonti, the Remingtonoceti, the Basilosauri. It was dealt of animals not still only aquatic but going to specialize. They lived in the so-called Sea of Thethys the body lengthened (from a pair of meters until 21 m) equipped of one long tail to whose sides had grown two lobes for increase the propulsion in water, posterior legs reduced or passings and the front ones transformed in fins, the nostrills moved towards the top of the head; the teeth still were distinguished in incisors, canines and molars; the auditory apparatus still very developed. They did eat mainly of crustaceans and fish that roused in low waters. Turtles, crocodiles, fish lived in their same coastal ambient fish-cat and molluscs. But they slowly began to specialize themself in the hunting to the wide sea, they were the Basilosauri, that had a snaked body. The Basilosauri can subdivide in two familys: Basilosaurinae and Dorudontinae. Just from these last ones it is thought (but the contrary opinions do not lack) have had origin the current  Cetaceans. Approximately 25 million years ago in fact all the Archeoceti disappeared after to have given life to two groups of Cetaceans, current the Odontoceti (from the Greek odontÚs "tooth", therefore cetaceans with the teeth) and Misticeti (from the Greek mystax "moustaches", therefore cetaceans with the whalebones). In the first ones teeth became uniform (omodontia) and development the use of the biosonar in order to characterize the prey; not all the Odontoceti born in those years survived until to our days the Squalodontidi and the Eurinodelfinidi as an example has been extinguished. Second ones (misticeti) born 34 million years ago constituted the result of the evolution of the dental apparatus that had been modified for the capture of small prey; the teeth will yield the place to whalebones (corneous sheets adapted to leak) and the alimentary habits forced to periodic migrations to the search of the food. In both cases however the evolution had carried to animals able to colonize all the seas of the world with a multitude of species variety. The molecular biologist have recently begun to study the evolution of marine mammalian confirming, through genetic tests, how much already supported from the evolutionary theses (have been found affinity as an example between the blood of the cetaceans and the one of cow, of the camel and the pig).  
As said initially, the study of the evolution of the cataceans is a puzzle whose pieces found are little and difficult to place in the correct place; it testifies the fact that many key points of what we said, also being supported from authoritative scientists, are from others refused (one for all the controversial positioning of the Sperm whale between the Odontoceti or the Misticeti).
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