Starting long before civilizations developed on Earth, and continuing into the early historical period, Martians raided the planet for human slaves. Humans were desired for their great strength and mobility, but the strength was soon lost as the Human children adapted to Martian or Mercurian gravity. Exercising maximized the potential, as Human musculature, skeletal structure, and ligament strength peaked much higher than Martian strength, even in those born on Mars, but the fast evolution always dissipated the potential after a few generations, so that the potential for greater strength was gone.
The raids continued long after a sufficient breeding stock was acquired on Mars and Mercury, in an effort to breed strength back into the species, but the resulting humans always eventually bred back to the same type, and the great strength was lost. After the Martian ecological crisis, when Mars withdrew her ships from spacefaring in order to conserve resources, the raids stopped, and evolution molded the Humans to a new form.
Mars Humans are very tall and slender. Their chests and lungs are larger for efficiency in the thin air, and their eyes are larger and better adapted to dimmer light. Their hands are long, with thin, spidery fingers. Their noses are longer and arched, and their ears are larger and more mobile, able to turn and cup to catch sounds in the thin air of their new homes.
Humans have long ago won their freedom from the Martians, and now live side by side with them. Their societies mirror their Martian models - Human Houses with Human Families, Martian Houses with Human Families, and Human Houses with Martian Families all occur on both Mars and Mercury. Since Humans are not usually born in Quads, their group marriages are artificial - consisting of singletons and the occasional twins. Multiple births are held to be lucky, and are greatly desired. They occur more frequently than on Earth in certain families, as the genetic predisposition to multis is concentrated in a few "lucky" families.
Mars Humans cannot move unaided on Earth or Venus. The marvelous powered exoskeletons that allow Martians and Mars Humans to visit heavy planets are uncomfortable for long periods, and so they avoid the "Bonecrushers", as they term Earth and Venus.