Paul was the first man on Mars as part of the Ares mission. They had sent him out alone and in haste, the Chinese were threatening to do the same. People complained that it should have been a woman, or a man of color but Paul was the best qualified to go there and come back. During the mission he took soil samples, to be studied on Earth. However Mars had once had water and life, before becoming the rust heap that humans knew. That was important, the alien parasite had attacked Martian life and dominated the planet before becoming the freeze dried wreck that we know today.
The Martians used a weapon to turn their world from habitable to that of a freezer. They ended their lives so that Earth might have a chance. They had frozen the parasite and doomed their own species. microbes released by the Martians took the hydrogen and carbon dioxide from the Martian atmosphere turned it methane. Methane then froze the planet, the opposite of the the greenhouse did on Venus. What the former owners of Mars didn't know was that Humans would one day send a man to go drilling into the ground. Paul took soil samples and loaded them into his rover to head back to the Ares.
In orbit around Mars, aboard the Ares, the parasite was awoken from a slumber of billions of years. Previous rovers sent by various countries didn't dig as deep as Paul did and didn't know what they were looking for. The parasite was hardy, having survived the depths of space on an asteroid being frozen was only imprisonment as long as it stayed frozen. Onboard the Ares it was human temperatures, the soil sample started to thaw. The sealing latch on the metal tube snapped open and the parasite hit the floor in a liquid like state. Once in range the slime like mass turned into a mass of black particles that entered Paul through his mouth, nose, anus and ears.
Paul collapsed onto the ground and went through a seizure as his human body was infected and taken over by the parasite. There was a several minute delay between Earth sent messages and Mars could take 30 minutes based on the position of the sender and receiver. However he was up before Houston got to worried. The man he was now was something more than human. The parasite had quickly detected his DNA and spliced itself to him. He now had one mission more important than all: go to Earth and create a new species to replace humans.