Reported abductees and contactees have consistently reported only a handful of alien types. The Reptilians, the insect like Mantis variety, the Nordics who look so much like us that they would not be noticed on the streets of Oslo, and the classic Grays. All of these have the same basic configuration that they share with us. Each has two arms and two legs. The head is placed on a moveable neck at top. Each has two eyes offering binocular vision that are placed on the head close to the brain. There are other similarities. Why should they resemble us so closely?
Skeptics point out that any being that evolved on another planet, or another dimension would have had an entirely different evolutionary history than us. They should then look entirely different from us sharing no characteristics at all. Certainly Science Fiction never had this problem. H. G. Wells began the tradition of truly alien appearance in The War of the Worlds. He described his aliens as about the size of a bear with an array of tentacles and a strange V shaped mouth. No arms or legs. More recent movie and TV Science Fiction has only deviated from this concept when cost and the need for actor recognition forced them to resort to makeup.
But reported genuine aliens continue to have the same basic configuration as ourselves. Why?
The basic process of evolution is random, the reconfiguring of genes and modifiers in the DNA molecule. The execution of the process is not random any more that the path of a drop of water is random after it falls as rain. The drop makes its way through aquifers, rivulet, streams, creeks and rivers to return to the ocean without any direction at all. Evolution creates species that fill niches, predators, herbivores, insect eaters, fliers and so on. Those which work survive and reproduce. Those which don't work, do not survive.
We see this every day. Occasionally a negative trait is introduced and medicine has to work to assure the survival of the individual. We call these genetic disorders. They have names like Huntington’s disease and Cystic Fibrosis.
The successful niches are filled by millions of species adapted to them. The shark comes in a variety of forms but each is so perfectly evolved as an aquatic hunters that they have remained in their current form for millions of years.
The Wolf is a mammal predator found over much of the world. It's a perfect hunter. But one continent it did not evolve on was Australia. There the niche filled by the wolf in North America, Asia, and Europe was filled by an entirely different creature, The Tasmanian Wolf. This predator isn't even a mammal like the wolf. Its a marsupial. Its appearance however is strikingly like a wolf. It has four legs for long distance running and rapid sprints to chase down prey, a highly developed olfactory sense, eyes and ears set high on the head that can be focused on a target. Its appearance is strikingly canine but it isn't even remotely related. It gains its appearance and characteristics from the predator niche it fills.
A sentient being fills a niche too. If it is to survive before it finally develops a technology to dominate its world it must stand upright to see potential predators, it must have hands to manipulate its environment, It must have binocular vision and have those eyes close to the brain to reduce time from vision to understanding. This list goes on. The basic upright form of humanity is as far as we know the best configuration for a sentient being and seems to have no evolutionary requirement except for the niche it fills.
The Troodon is a small bird like dinosaur from the Cretaceous period. In 1982 Dale Russell, curator of vertebrate fossils at the National Museum of Canada in Ottawa decided to theorize what this dinosaur might look like if the killer asteroid had not hit and it had been allowed to evolve into a sentient being. What he came up with looks strikingly like the lizard beings and a little like the Grays yet Russell used only evolutionary logic and had no interest in UFOs whatever. He extrapolated a being that fills a niche, presently occupied by us, and then conjectured how it must look.
It should not therefore come as a surprise that upright bipedal beings are being reported aboard space craft. They are probably more common than we imagine.