A brief history of life
Life in the broad sense is fairly common. Microbes can be found living nearly anywhere there is liquid water and a source of energy. They arise spontaneously under suitable natural conditions in a relatively short time span on geological scales. Near-biotic complexes undergo rapid optimization to a local fitness maximum that incorporates a near-universal biochemistry base, including a lipid bilayer membrane, proteins assembled from the same set of amino acids, double-stranded DNA as a template for assembling proteins, RNA serving an intermediate role in DNA transcription, ATP for energy transfer, proton pump respiration, and metabolism of fats, sugars, and amino acids.
Even the chirality of amino acids and sugars is specified in this fitness maximum due to minor effects of the weak nuclear force on chemistry.
The near universality of life can be obscured, however, by the artificial transport of living organisms that occurs when a sapient species discovers wormholes and spreads through space, a process known as panspermia.
Two major groups of microbes, the archaea and bacteria, are found throughout the Local Group (and perhaps beyond), and nearly all complex organisms are the product of a long-ago symbiosis of these two groups to produce eukaryotic cells. Life forms beyond these basic classes can be found, but typically only in isolated refugia within a sea of bacteria and archaea.
On Earth, the remains of clades of life from indigenous abiogenesis are evidenced by the presence of certain varieties of virus, adapted to a parasitic role after being out-competed for other niches by the alien microbes.
The most well-known of the panspermia events in the Local Group of galaxies occurred between -18Ps to -16Ps CNT (570 to 510 million years ago), when the Antecessors were active. Where Antecessors went, animals and fungi followed. Escaped pets or pests, in the form of worms and bugs, became established on worlds throughout all galaxies so far explored. On Earth, this invasion was known as the Cambrian explosion, a sudden proliferation of complex life in the fossil record. The native Earthlings – gelatinous, sessile, quilted mat-like creatures called Vendobionts – were quickly eaten into extinction by the more advanced true animals that took over the planet, and a world-wide ecosystem of bacterial mats was (literally) overturned by burrowing action of the animals.
It is noteworthy, however, that the Vendobionts were not the only native Earthlings. Another group, the plants, evolved independently on Earth as well and came to share the planet with their invasive animal neighbors. Other worlds have no true native plants. Some will have evolved their own large, complex autotrophes (creatures that synthesize their own food from ambient energy present in the environment). On others, animals or fungi will have evolved to take the autotroph role - usually by undergoing symbiosis with some variety of photosynthetic microbe. Since the expansion of Humanity throughout the Milky Way galaxy, however, plants have been spread far and wide.
There is no evidence of further panspermia events involving Earth after the disappearance of the Antecessors until the expansion of Humanity into the galaxy. This means that while many worlds will have chordates and arthropods, only Earth has vertebrates or insects or arachnids or crustaceans, except insofar as those phyla have been spread by Humans. Despite the hugely popular flights of fancy propagated by entertainment media, there is nowhere in the galaxy that you will find non-avian dinosaurs or alien cat-people that are curiously compatible with Human reproductive biology.