It had long been speculated that sexual intercourse and internal fertilization had their beginnings in the lineage of cartilaginous fishes that includes sharks and rays, otherwise known as the chondrichthyans. Prior to the evolution of this feature, sea creatures all reproduced by spawning: female fish simply released their eggs into open water, after which the male fish swam by and fertilized them, leaving the embryos to develop outside the bodies of the parents. Many fish and amphibians, in fact, still reproduce in this way.
But the evolution of internal fertilization was a huge milestone in the history of life on earth. Even though this method produces fewer offspring, the offspring produced are larger and more likely to make it to adulthood, which would have been favorable from the standpoint of natural selection. Recent finds of fossilized placoderms demonstrate that internal fertilization evolved much earlier than previously thought, in a lineage of plated fish that are direct ancestors of tetrapods, and hence of human beings.
Australian Fossil Finds
In 2005, researchers were looking for fossils in northwestern Australia, in an area that 375 million years ago was a tropical reef. Several specimens of fish known as placoderms ("plated skin") were found, which was not in itself remarkable; these early jawed fish were common creatures in late Devonian-era seas, and dominated their environment for over 70 million years. Upon closer examination of one of the fossils, however, something amazing came to light.
Pregnant Placoderm
The fish was a previously unknown species of placoderm, which was exciting enough by itself. But the most extraordinary thing about the fossil was that it carried an embryo, complete with mineralized umbilical cord. The 375-million-year-old fish was now the oldest example of a creature that had sexual intercourse and incubated its offspring inside the mother's body. The new species was dubbed Materpiscis attenboroughi, literally "Attenborough's mother fish."
The Evolution of Reproduction
Since Materpiscis and related placoderms are on a line of direct descent to humans, researchers are interested to study the rudiments of the fishes' reproductive system in order to determine how and why sexual organs evolved as they did. They theorize, for example, that the fishes' jaws may not have evolved for chewing originally, but for holding females in place to ease copulation, with the jaws only being pressed into service for chewing much later down the line.
Additionally, the pelvic fins known as claspers — which are used to deposit sperm into the female and which some fish including sharks still possess — may have eventually evolved into legs and genitalia in the lineage leading to tetrapods and eventually humans. Lead researcher John A. Long also points out that there may have been a transition period between reproduction by spawning and internal fertilization which may have involved the embryos developing in some sort of pouch, much like seahorses employ today.
Source:
Long, John A. "Dawn of the Deed." Scientific American Jan. 2011: 34-39. Print.
Jenny Ashford - Jenny Ashford is a writer and graphic artist from central Florida. Her main area of interest in her Suite 101 articles is science, with a ...
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