What do we know about the evolution of sex? Specifically, what do we know about the process of internal fertilisation of eggs by sperm?
Information relevant to these key questions have been documented in a new discovery that was recently reported in the journal Nature and discussed in an online article by the BBC.
The discovery actually happened in the laboratory when a group of scientists at London's Natural History Museum began investigating Placoderms, a group of fish dating from the Devonian ("the age of Fishes), that were characterised by a strong external armour-like covering. One particular specimen, dating to approximately 365 mya (million years ago), shows particularly striking and direct evidence for internal reproduction. A small embryo, 5 cm in length, was found inside this particular fossil. Internal fertilisation is also suggested among the Placoderms, in general, by evolutionary changes to the pelvic fin which forms a structure known as a clasper. The anatomical modification provided a means for male and female fish to connect - so to speak - during mating.
The Placoderms were among the earliest jawed vertebrates. thus occupying a place in our evolutionary heritage near the origins of vertebrates. Placoderms - like most species, both ancient and modern - became extinct. They were suceeded by a group of bony fishes that ultimately evolved into the tetrapods - animals with a four-limb pattern that include birds, mammals, reptiles and amphibians.
I'll have more to say about the evolution of animals and plants in upcoming posts on variation and intermediate forms.
Thanks to Graeme Wright for information about the findings pertaining to Placoderms.