Clades adapted to microgravity often need mechanical assistance when visiting planets or locations with artificial gravity
The human form evolved specifically to thrive in an Earth-like environment. When humanity began to colonise space, it was apparent that environments resembling the land surface of the Earth were in fact vanishingly rare. For this reason artificial environments were built in space, resembling Earth as closely as possible; the absence of gravity in freefall was a particular problem, one which could be remedied by rotating space habitats of various kinds. At the same time elaborate plans were made to terraform Mars and other bodies in the Solar System and elsewhere, with the aim of recreating the environment of the Earth on the surface of alien worlds.
But increasingly sophisticated genetic engineering technology at this time promised a different strategy; instead of adapting the environment to suit the colonists, they could themselves be modified to suit the environment. This process already had a name; pantropy, coined by Atomic Age writer James Blish, meaning a process which changes everything.
Pantropy can be achieved by a number of different methods;
Cyborgs, biological sophonts augmented with artificial components
Tweaks; biological clades with minor or major adjustments to their genome
Neogens; with entirely new genomes designed from scratch.
Xenogens; with genomes and biochemistry based on local examples
In the Interplanetary age a number of humans with prosthetic body parts volunteered to have specially designed bodyshells which would allow them to operate in extreme environments. Cyborgs of various sorts were involved in the colonisation of Luna, as asteroid miners, prospectors on Venus and on the moons of the outer planets. Since that time many worlds have been initially explored by a wide range of cyborgised bionts.
Early Genetic Tweaking
The first humans genetically adapted for a non-Earthlike environment were the Homo cosmoi; these new humans could survive indefinitely in freefall, and had some resistance to the increased radiation levels encountered in outer space.
Another very early adapted form of humanity were the Martian tweaks, capable of living for extended periods on the Martian surface in the very low atmospheric pressure there. However the terraformation of Mars had already begun; over time the Martian tweaks were displaced from that world, as the atmospheric pressure increased and made their adaptation unnecessary. The extraordinary expense involved in changing the Martian environment was seen at that time preferable to the idea of creating a race of humans who could only live comfortably on the surface of a nearly airless planet.
When humanity spread to the stars, a great range of planetary environments was encountered; but none exactly replicated that of the Earth. The somewhat higher gravity of Nova Terra and Penglai made mild pantropic adaptation necessary; on other worlds colonists were genetically altered to be tolerant of trace elements or variations in the stellar spectra. Even on Mars the terraformation process was never completed, and the population of that world today are all descended from people who have been altered to live comfortably in relatively low pressure.
An adaptation to an aquatic lifestyle was a very early form of pantropy that has become very widespread. Water is a very common compound in planetary environments, although it is generally locked up as ice. Heating an icy world, either locally or on a global scale, provides an environment that is suitable for water adapted clades like the Merpeople (or water-breathing clades like the Europans and their descendants). Very often the water has dissolved salts or other factors which require the waterdwellers to be modified still further.
Note that not all tweaks are designed to live in environments which are not Earth-like; many have been modified to occupy quite Earth-like environments, but not those which can be inhabited by nearbaseline humans. Examples include the aforementioned aquatic tweaks, flying tweaks, those adapted for tree-dwelling or to underground environments and so on. The Genetekker civilisation, and the later House Genen which emerged from it, have a long history of specialisation in genetic modification or various environments.
A prospective colonist who desires to become adapted to a new world may use a number of different pantropic techniques. The most basic is somatic gene therapy. Here genes are introduced by virus or nanomachine into the cells of an individual. This is adequate for low-level changes such as tolerance of different gravities or waterdwelling adaptations. Somatic therapy can change a baseline biochemistry to a limited extent, enough to produce tolerance of certain alien proteins, or a moderate change in ambient temperature.
But to radically change an individual's physical form, or biochemistry, or to allow tolerance of extreme temperatures, radiation or pressures, that individual's body must be completely rebuilt. Bionano-assembler technology can achieve this in certain cases, although this process is generally far too traumatic to occur while the individual is conscious. To adapt the colonist to a completely novel environment the best way is usually to upload that person's consciousness into virtual form, then download them into a newly constructed Neogen or Xenomorph body. Specialised bio-engenerator equipment is required for this process, which can take several days. Alternately the uploaded consciousness can be downloaded into a robotic body designed to withstand the conditions of the new world or environment.
Once the colonist has a new form, germ-line modifications will allow em to breed true; any offspring e may have will also be adapted to the new environment (allowing for normal genetic drift). In the case of colonists downloaded into artificial bodies, in many cases these bodies can be made self-replicating (effectively becoming von Neumann machines). Sometimes the new forms, either biological or artificial, cannot easily be made to replicate themselves, and must be grown or manufactured industrially.
Extreme environments and Xenobiochemistry
But other environments required much greater changes to be made before the population could thrive. On the planet Trees, for example, a complex biosphere already existed, which differed biochemically from Earth life quite radically. To allow colonists to live on this world they had to be radically changed at the molecular level, with many proteins and metabolic compounds being replaced with native versions. This adaptation allowed the colonists to digest local foodstuffs, but such a radical adaptation took many centuries of research and development, and many tragic failures, to achieve.
Extreme environments such as the hydrogen atmospheres of gas giants, and the cold surfaces of ammonia-ice worlds, required even more radical biochemical adaptations. In some cases, the discovery of alien life in an environment entirely different to that of Earth gave impetus to the development of pantropic adaptations suitable for such conditions. For instance the floating ecology found at 54 Hydrae (Ruach) consisted of life where the cytoplasm was gas-based rather than liquid based. After this discovery a range of very low density clades were created, capable of floating in the atmospheres of similar Jovian worlds. Similar discoveries of very-low temperature ammonia- and methane-based life allowed the development of low-temperature tweaks and neogen clades, using similar biochemistries.
When the high-temperature biosphere on To'ul'h was discovered, human ambassadors and xenologists were radically altered to tolerate the searing temperatures of that world. At first these adapted humans were given humanoid forms, despite their extremophile biochemistry; however the conditions on the surface of To'ul'h are so different to anything that a human form can accommodate, these To'ul'h-adapted humanoids were practically helpless in the thick, hot, flowing winds at the bottom of the atmosphere. It was not until visitors to To'ul'h were given forms resembling the local biota that they could begin to function in those conditions. Similarly those To'ul'hs who came out of their shrouded planet to meet with Terragens in their own society were entirely helpless until they were given suitable forms (generally, although not exclusively, humanoid).
Ethical aspects of Pantropy
The radical changes associated with pantropy can lead to radical changes in the mentality of the colonist. Once given a new biochemistry, or an artificial body and mind, the functions of the colonist's brain and mind may be subtly or radically altered. From the very outset of interstellar colonisation there has been a debate over the ethics of altering human bodies and mentalities in this way; although the inhabitants of extreme worlds may be descended from Terragen ancestors of one kind or another, they are often so changed to be almost entirely alien.
Sometimes the process of extreme modification required for pantrophic adaptations produce an entirely novel form of modosophont, entirely different in mind and body from the species from which it was derived. For instance the cold-adapted Methanoid clades use a xenogenetic template as a basis for their biology, and thrive at very low temperatures and low subjective speeds. Even though Methanoids are humanoid, and resemble baseline humans in many respects, they cannot tolerate temperatures above 100K and rarely interact culturally with warm-modosophonts. When the Methanoids were created some questioned the wisdom of creating such an extreme species, likely to become isolated from mainstream civilisation.
In fact many conflicts have developed between species modified by Pantropy and those who attempt to create an environment that more closely resembles that on Old Earth. The expulsion of the Martian Tweaks, similar expulsions of high temperature tweaks from Ribblehead and Venus, and various conflicts between Methanoids and warmer and faster clades culminating in the Epp War are examples of this sort of dispute. However the existence of so many planetary types in the galaxy means that in general there is plenty of room for a wide range of clades, each adapted to a different environment.
To what extent the diverse inhabitants of the Terragen Sphere can claim to be descended from lifeforms originally found on Old Earth is highly debatable, and for many of those inhabitants the question is practically irrelevant. Each citizen of a new colony has a unique perspective on reality that is tempered by their physical form, no matter how exotic, and the days when baseline humans were the only significant members of Terragen Civilisation are practically forgotten by all but a few.
Homorph - Text by M. Alan Kazlev Any non-human, such as a transhuman, posthuman, vec or alien, using an artificial humanoid body.
Homorph, Alien - Text by Steve Bowers Xenosophonts who deliberately adopt human (or general terragen hominid) form, morphology, or biochemistry. While homomorphism is fairly popular among some alien races and clades, many alien societies are under embargo to avoid cross-cultural pollution, so the homorph is often illegal. See also Toulhuman.
Space People - Text by M. Alan Kazlev  (archaic) A common early term for Space Adapted Humans, still used by some descendants of the original clade.  (Second Federation Era) A term for vacuum adapted tweaks, now rarely used.
Space Spiders - Text by Steve Bowers Vacuum and zero gee adapted, sentient or semisentient spidersplices, from micro-scale to giant; usually capable of producing buckyfibre silk. Contribute to many megascale building projects, sometimes controlled directly by transcended postspiders.
Terrachauvinism - Text by M. Alan Kazlev Belief that Terragen life and intelligence is superior to other lifeforms and intelligent beings in the galaxy, and that this is the reason why the Terragen Bubble is the largest civilization at present. A sort of variant Anthropism that includes all Terragen mindkind. Terrachauvinism completely ignores extinct alien civilizations of greater than Terragen extent, as well as evidence of large empires detected by the Argus Array.
To'ul'human - Text by Steve Bowers Homorph To'ul'h who have been given or adopted human shape by advanced biotech. They also can take To'ul'h form in earthlike environments or adapted human form in toulovenusian environments.