Arkhammers - The Miskatonic University was a leader in the field of Occult Science before the fall of Earth. When Earthgov turned inwards in desperation and violently cracked down on evacuation, 9 Miskatonic professors opened an illegal Gate to bypass the blockade and brought their families and brightest students through. For ages they were without any home but the ships they bought or salvaged, wandering from station to station trading scraps of knowledge and expertise for resources. They grew insular as fear and suspicion followed them but they continued to pioneer new occult techniques, including the domestication of Hyperspatial lifeforms. Their expertise payed off with the end of the Boojum war and their aid in coming to an agreement with the Leviathans. Now their Dean holds an honored place in the Solar Council and their Archive Ships are celebrated places of Occult learning.
Meat Witches - The Meat Witches are an offshoot of the Arkham Collegiate, throwing themselves deeper in their study of Hyperspatial life-forms and self modification.
Spacers - It is perhaps unfair to lump most of the human population together simply because they all live in space but without that single unifying element there would be very little in common. Habitats litter the system, everything from O'Niell cylinders that house tens of thousands to simple mircograv cans with small family groups. With the distances involved it is nearly impossible to stay united and culture can vary wildly from hab to hab. This individual is a member of the Sol Defense Fleet, the military arm of the Solar Council.
Martians - Mars was the first planetary colony to declare independence from the old Earth government, all the way back in 2282 or -19.7decasol depending on your choice of calendar. Mars itself has been terraformed for thousands of years and is now a world of cold rocky deserts and a vast northern ocean. The martians are a fiercely independent and individualistic people, valuing solitude and self sufficiency over comfort and community.
Venusian - Unlike Mars, Venus was not terraformed by good old fashioned comet drops and algal blooms. Ancient technology mistakenly activated by surveyors remade Venus into a world of steaming jungles and saltwater marshes practically overnight. Venus is seen as a world of rebellion, having torn the Hypercapitalist oligarchs that once ruled their planet from their ivory towers and fed them to the swamp lizards.
Brainguys - These posthumans designed themselves to be the perfect vessels for psionic energy. They have a few habitats in the Asteroid Belt but they rarely spend time active in realspace, preferring to spend their time meditating and experimentation on building their own Dreamlands.
Starmothers - The Starmothers are a mystery. These Posthuman entities wander the inner solar system, floating through space under their own power. Occasionally they will set down on a planet for a period ranging from a few hours to months at a time. They aren't hostile but no attempt to communicate has been reciprocated. Some attempted conversationalists have reported that they felt an unnerving sense of attention from the enormous fetuses floating within their bellies.
Silver Lodge Hunters - The Hunters are descendants of a fraternal secret society of sport hunters that discovered the occult in the late 20th century. Even before the rising of R'lyeh in the 23rd century their familiarity with outer forces was beginning to change them. Now they are something like a ghoul, built for apex predation rather than scavenging. They are not well liked as they are happy to hunt other transhumans if the mood takes them but they are also happy to throw themselves into monstrous or alien threats with the right encouragement. Even the Mi-go make use of them from time to time, when they have need of a convenient thug.
Digital Ghosts - Full mind-machine uploads are relatively unpopular among humanity as continuity of consciousness is easier to maintain with Occult Science and Psionic transfers than it is with physical science. However it is not without its own benefits and they have built up a fairly robust population of distinct personalities over the last few thousand years. Most of their population is housed in server-bunkers buried in Triton's ice and if they have to interact with the rest of pan-humanity they generally do it via telepresence.
Encephalon - The Encephalon are descendants of transhumans who where disgusted by what they felt where petty politics of useless meat. They hold to the ideal that the only thing that matters is the brain, so they removed almost everything else. They despise being compared to the now jarred humans that joined with the Mi-go colony, despite the inherent similarities in life-style. For one thing their somatons are cubes not cylinders!
Church of the Perseverant Machine - Despite their fearsome appearance, it is difficult to find gentler souls than those of the Perseverant. In the years following the fall of Earth, religious practices where suppressed but the Church never disbanded or apologized for their beliefs and met mistrust and oppression with kindness.
This is hardly a fully inclusive list of the extant sub-species of humanity but it is a decent cross-section of the biggest players and some of the weirder outliers.
Some colonists have even gone out beyond Neptune and into the Void beyond but they have no contact with the Sol system.
Mars is all imports from Earth and a few completely artificial lifeforms cooked up in martian geneforges. It's mostly pet animals, livestock and feral versions thereof. Mars' biosphere is not terribly diverse. The only real exception is that martian cats sometimes leave the Dreamlands to play with waking cats.
Mars also has native but extinct life that I haven't put too much thought into yet.
Venus has a very thick atmosphere with plenty of oxygen so it has megafauna all over the place, from gigantic insectoids to pseudo-dinosaurs.
I keep meaning to post that as part of a doodle page. It was super quick and I feel weird about posting stuff like that alone.
There have been a few physical visits from the spacer humans, mostly anthropological expeditions by Arkhammers but Earth is considered too dangerous to visit frequently. The Silver Lodge has also gone on a couple Predator-style murder sprees but the Tsan Chan don't hold it against anyone else.
Overall I'd say they're cordial but distant.
I've never read Man after Man, although there are similar post-humans in All Tomorrows. I figure turning yourself into a giant brain is a pretty logical route for transhumans. Especially in a universe were psionics work.
I actually have a reason for the teeth with the Perseverant though! In the early days they'd paint smiles on their carapaces but over the last few thousand years the stylization mutated until it basically became Teeth! Teeth! Teeth!