How did the mammals & other species develop into 2 different Sexes ?
Since evolution’s pace is really slow , there must have been a rather long interval of time before mammals were able to develop the first steps of the reproductive system , how did they survive this interval ? Did they go on with the asexual reproduction ?
If Archaeologists found any fossils explaining that , I will more than glad to read/see the article if you got a hold of it [i.e. post the link Please XD] .
Mammals were descended from reptiloids that had two sexes.
Those reptiloids were descended from amphibians that had two sexes.
Those amphibians were descended from fish that had two sexes.
Those fish were descended from nonvertebrate chordates that had two sexes.
Those nonvertebrate chordates were probably descended from neotenous echinoderm larvae that had two sexes.
Those neotenous echinoderm larvae were probably descended from hermaphroditic echinoderms — a couple of mutations gave ‘em neoteny and separated the sexes.
Mammals were descended from reptiloids that had two sexes.
Those reptiloids were descended from amphibians that had two sexes.
Those amphibians were descended from fish that had two sexes.
Those fish were descended from nonvertebrate chordates that had two sexes.
Those nonvertebrate chordates were probably descended from neotenous echinoderm larvae that had two sexes.
Those neotenous echinoderm larvae were probably descended from hermaphroditic echinoderms — a couple of mutations gave ‘em neoteny and separated the sexes.
References :
mammals evolved from a reptile, which in turn evolved from an amphibian, and the amphibians evolved from a fish. Fishes evolved from more primitive chordates, which did not have any back bone, only a notochord. Chordates first appeared during the Cambrian explosion, some 530 million years ago. But even before that, sexual reproduction has already evolved:
"All sexually reproducing organisms derive from a common ancestor which was a single celled eukaryotic species. Many protists reproduce sexually, as do the multicellular plants, animals, and fungi."
http://en.wikipedia.org/wiki/Evolution_of_sexual_reproduction
That means, when the first chordate evolved from a single celled organism, sexual reproduction was already present.
BTW, the Cambrian Explosion is positive proof that evolution is not necessarily slow. Scientists are still debating how the Cambrian Explosion happened, and there are som who deny that there was an explosion, but the fossil record is unambiguous. Most likely the cambrian explosion was made possible by the evolution of Hox genes, which enabled the appearance of a wide variety of body plans. Once organisms evolved into one of those body plans, they are stuck and cannot go back to evolve other body plans. It is evolution’s equivalent of "you cannot go home again." Conditions have changed since the Cambrian explosion, and nature cannot regenerate the same conditions that enabled those body plans to appear in a geological instant.
As I said, mammals evolved from a reptile, and reptiles were the first vertebrate animals to evolve a penis. Some living reptiles (e.g. the tuatara, a lizard-like reptile from New Zealand) do not have a penis. The reptilian ancestor of the mammals therefore probably evolved a penis, which enables internal fertilization. Most reptiles have internal fertilization as well. Most reptiles lay eggs, just like the primitive mammals called monotremes, i.e. the duckbilled platypus and the echidna. Many reptile lineages have evolved live birth independently from one another by retaining the eggs within the female’s body until they have hatched. The marsupials have apparently evolved the same independly of the reptiles. The placental mammals went one step further and evolved a placenta, from which the fetus obtain nourishment from the mother. Therefore during every step of the way from external to internal fertilization, from egg laying to retention of eggs, and from egg retention to placenta, the mammals were reproducing sexually without interruption. There need not be any asexual reproduction stage in the evolution of mammals.
References :