One of the greatest mysteries of the known galaxy is why intelligent species on so many worlds have evolved in such similar ways. There are literally thousands of races which have two arms, two legs, a torso and head, whose sensory organs operate in a similar manner, who even breathe approximately the same gas mixture. 
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That this could happen by chance is so unlikely as to be virtually impossible, and many outlandish theories were advanced to explain the anomaly.
In 2369 Professor Galen, a Federation expert in Archaeology, was killed whilst investigating a mystery involving the DNA patterns of many different species. Captain Picard and the crew of the Enterprise-D followed up Galen's research and discovered that ancient DNA strands from many different species could be combined to form a message. Picard attempted to find the missing pieces of the puzzle, finding himself involved in a chase with the Klingons, Cardassians and Romulans. During a tense stand-off with the competitors, who thought that the message might contain plans for some advanced power source or weapon, Picard was able to retrieve the final piece of the puzzle from a section of fossilized seabed on a now uninhabited planet.
The message was a computer program which reconfigured Dr. Crusher's tricorder, projecting an image of a humanoid alien. The being, apparently a female, informed the astonished onlookers that her species had evolved billions of years before. Finding themselves alone in the galaxy, the species had feared that if they were to die out they would pass away without leaving any lasting imprint on the galaxy. They decided to seed the primeval oceans of many worlds with DNA codes which would direct the evolution of life on those worlds towards a pattern similar to their own. In this way the aliens hoped that they would gain some degree of immortality through their 'offspring'. The message also expressed the hope that their progeny would have to work together in order to access the various DNA strands.
Even after this discovery, surely one of the most momentous in all of history, little is known of the Progenitor species. We do not even know the name of their species or how far they spread themselves, although the presence of humanoid species in both the Gamma and Delta quadrants indicates that they were likely a galactic-scale power. It is possible that the Progenitors left further messages in the DNA of other species, and much research into this area has been done since the discovery of the first message. Unfortunately, nothing has come of this effort.