Astrobiology

Has extra-terrestrial life evolved in a similar way to life on Earth or did it evolve differently? This assumes extra-terrestrial life exists.

The earliest evolution
There are numerous features shared by many and often all cellular organisms, features that indicate a single origin for all the Earth's present-day biota: The Last Universal Common Ancestor (LUCA), at least of the prokaryotes, is reconstructed as having had all of these features.
 * DNA for master copies of genetic information
 * RNA for intermediate copies and various other functions
 * Proteins for a variety of functions: enzymes, structural materials, etc.
 * A shared set of 20 protein-forming amino acids
 * Lipid-bilayer membranes
 * Chemiosmotic energy metabolism: pumping hydrogen ions outside of the cell, then making them assemble ATP molecules in their return
 * Electron-transfer energy metabolism: transfer from various electron sources to various electron sinks, typically coupled to chemiosmotic energy metabolism
 * Shared asymmetries in their molecules

Many of the smaller amino acids, and various other sorts of molecules, like porphyrins, can be produced prebiotically, and they may thus be common. However, the fancier ones are likely the result of various historical accidents, and are thus unlikely to be duplicated.

One can also reconstruct a lot of pre-LUCA evolution, like an "RNA world", where RNA did the functions of DNA, RNA, and proteins. Some coenzymes got their start back then, like several of the B vitamins, and proteins likely originated as coenzymes that were expanded to become the primary enzymes. Ribosomes, protein-assembly structures, are essentially RNA enzymes (ribozymes) assisted by proteins.

The main problem with the RNA-world hypothesis is the origin of the RNA. The nucleobases (adenine, guanine, etc.) can be formed prebiotically, but ribose is a 5-carbon sugar, and it's very difficult to form sugars prebiotically. The Butlerov formose reaction can do so, but it requires concentrated formaldehyde, and it produces sugars with a variety of sizes and the full range of asymmetries. This has led to speculation about ribose having replaced some alternative, like amino acids or polycyclic aromatic hydrocarbons. That may mean that ribose was a historical accident, and that extraterrestrial organisms may use different backbone molecules if they use a nucleic-acid-like template heredity system.

Energy metabolism
The LUCA and other early organisms were likely chemoautotrophs, subsisting on chemical reactions and making all their biological molecules from simple ones. They likely extracted electrons from sources like hydrogen and iron and sent them down their electron-transfer chains to sinks like nitrogen oxides and sulfates. Methanogens do a similar sort of metabolism, combining hydrogen and carbon dioxide to make water and methane, though the details are different, and their mechanism is specific to them.

Chemoautotrophy is rather limiting, since it makes energy metabolism dependent on molecules that may be hard to find.

However, there's another source of energy that organisms can and do tap: sunlight. In fact, photosynthesis has evolved twice. Bacteriorhodopsin photosynthesis pumps hydrogen ions out of the cell for chemiosmotic energy metabolism, and it's used by halobacteria and some other Archaea. Chlorophyll photosynthesis uses electron-transfer energy metabolism, but with chlorophyll molecules energizing the electrons in photovoltaic-cell fashion. It evolved once in Eubacteria, and it has had a complicated history of lateral gene transfer and/or loss there.

For biosynthesis, it is necessary to have an electron source; the electrons then combine with hydrogen ions from the surrounding water to make hydrogens in molecules. In fact, electron transfer for biosynthesis likely dates back to the RNA world. But some chlorophyll photosynthesizer worked out how to use a very difficult electron source: water. This enabled organisms to spread to environments that would otherwise starve them, but it was at the price of releasing a rather toxic gas: molecular oxygen. It evolved only once on our planet, in the ancestor of the cyanobacteria.

What makes oxygen toxic, its being a good electron sink, is what makes it useful for extracting energy from other molecules. As a result, several organisms have evolved oxygen-using metabolism, usually as the last step in electron-transfer energy metabolism. The proliferation of cyanobacteria made a certain electron source much more common: biological molecules. Thus, heterotrophy / organotrophy became much more feasible.

Eukaryotes
Though the prokaryotes have a deep split between Eubacteria and Archaea, they are still outwardly very similar. Eukaryotes have much more complexity, much of which has rather obscure origins. Mitochondria and chloroplasts are descended from alpha-proteobacteria and cyanobacteria, respectively, though the rest of the cell's ancestry is more obscure. Informational systems are most closely related to Archaea, while metabolic systems are usually closer to Eubacteria.

Eukaryotes can manage much larger genomes than prokaryotes can, thus enabling complex multicellularity. Prokaryotes range from 0.5 (parasitic) and 1 (free-living) to 10 million base pairs, while eukaryotes range from 3 million (parasitic) to a few hundred billion base pairs, though the larger genomes are filled with "junk DNA" (Prokaryotic Genome Size). Since the origin of eukaryotes is obscure, it may be difficult to tell how evolvable large-genome management is.

Eukaryotes also have full-scale sex, instead of prokaryote "sex", which is injecting snippets of genetic material into each other. It consists of this cycle:

Haploid phase - cell fusion - diploid phase - meiosis - haploid

Eukaryotic cells can do mitosis, ordinary cell division, in either or both of the haploid and diploid phases. An illustration of the complexities one may get is in the evolution of land plants. They alternate between a haploid gametophyte and a diploid sporophyte, named for whether the phase produces gametes or spores. However, seed plants do not go as far as most animals, where only the gametes are haploid.
 * Nonvascular plants: gametophyte most prominent
 * Primitive vascular plants: sporophyte most prominent, gametophyte reduced
 * Seed plants: sporophyte nearly all of the plant, gametophytes vestigial

The origin of sex has been much argued about, since from a genetic-selfishness viewpoint, it is deleterious. However, it is useful in making genetic variety, like for countering the evolution of parasites. So extraterrestrial organisms may also practice various forms of sex.

Multicellularity
The lines between one-celled, colonial, and differentiated-multicellular organisms can be difficult to draw, but differentiated multicellularity has evolved several times, in prokaryotes as well as in eukaryotes. However, most instances of it are either plantlike or funguslike, with animallike multicellularity evolving only once, in Metazoa, the animal kingdom. A curious hybrid has evolved several times: slime molds, which alternate between an animallike one-celled phase and a funguslike spore-making fruiting body.

Multicellularity mechanisms, like development-control mechanisms, have gotten the most study in the animal kingdom, and oodles of multicellularity-mechanism homologies are now known across much or most of the animal kingdom. Mechanisms like Hox genes, which are involved in nose-to-tail patterning. If someone discovered that some obscure tiny worm is closest to some distant big eukaryote group like Amoebozoa or Alveolata or Stramenopiles or Rhizaria or Excavata, that would be big news, but it has yet to happen.

So might other planets have lots of plantlike and funguslike and slime-mold multicellular organisms but no multicellular animals? That would make it difficult for sentience to evolve, since that is likely the result of having to evolve the neural processing capability to interpret complex sensory data, to navigate and utilize complex environments, and the like -- and to do so relatively fast.

Senses
Organisms must sense their environments, and they have evolved a variety of senses, for detecting:
 * Light (vision)
 * Pressure (touch)
 * Sound (hearing)
 * Chemical composition (smell and taste)
 * Temperature
 * Electric fields
 * Magnetic fields
 * Orientation (proprioception)
 * Acceleration and gravity
 * Pain

Interpreting visual information can be very demanding, and animals with high-quality vision often have much of their brains dedicated to visual perception. Doing echolocation can also be demanding; much of dolphins' brains is dedicated to interpreting what they hear. So we can expect sentient extraterrestrials to either have very good vision (like us) or very good echolocation.

Skeletal and limb features
To grow large on land, it is desirable to have an internal skeleton. However, true internal skeletons likely originated only once, among vertebrates, while most other animal skeletons are either external (shells), molted skin (in arthropods), or just under the skin (in echinoderms). So it may be hard to evolve an internal skeleton.

However, limbs are common, though it is uncertain how many times they have evolved. They are essential for traveling on land faster than a crawl, and sentient extraterrestrials would likely have limbs.

Grasping organs are necessary for manipulating one's environment, so sentient extraterrestrials are likely to have some of these, especially grasping limbs. They are also fairly common. Jaws evolved at least twice, in arthropods from limbs near the mouth, and in vertebrates from the first gill bars folding forward. Tentacles evolved in cnidarians and cephalopods, and some arthropods and vertebrates can use some of their limbs for grasping. Among arthropods, scorpions and several crustaceans have pincers on some of their limbs, while mantids press their front walking legs together. Among vertebrates, primates and perching birds have grasping limbs.