When examining a living human body you discover that a great deal of heat is emitted. All humankind's body temperature is 98.6F/37C. When the body dies it feels cold to the touch. In fact, it isn't. In reality, it drops to room temperature. The human body is made up of molecules, molecules that generate energy. The heat generated comes from digested food. It is the fuel which drives the human body. The ancient Chinese called this Chi. Chi is but a word yet whatever it is called that energy is what drives life. All life.
When we see images of extraterrestrials that we are led to believe are either living on or visiting planet Earth, you can be certain they are biologically like us. The same natural law would also apply to them. They too would have to comply with the laws of biology. For them to co-exist or just visiting they must have evolved from a planet very much like ours. To breathe our atmosphere they would need lungs, a heart that pumps blood, a liver, spleen and all the other organs that make what is effectively an advanced primate species.
Would they have to be a form of primates? I guess not. I suppose it is possible they may have evolved from another species of biped - maybe kangaroo's or lizards. Birds too are bipedal. One argument against this would be brain size. Birds and lizards, even kangaroo's, do not possess large brains. In this world, bipedal's rule. That shape offers great mobility. It enables us to hunt, fish, swim, run, walk etc; It makes perfect sense that the supreme species on Earth not only has the biggest brain but also a body that allows for great diversity of movement.
The aliens portrayed in the many images seen on the internet look remarkably like hominids. Same bipedal shape with arms, legs and an even larger head to fit an even larger brain. Maybe we homo sapiens didn't evolve here. Maybe we were brought here via an armada of spacecraft that arrived here having first located a planet best suited to support living creatures born on a water-based world, a world so like ours that ours is a virtual facsimile. This, of course, is not only speculation but also runs in the face of science. Now, you either believe in television, telephones, satellite dishes, computers, mobile phones, stereo players, videos, hearing aids, the control of diabetes, winning the battle against cancer, space labs, rockets to the moon, submarines, aeroplanes, hovercraft, heart surgery, WiFi, the cure for smallpox, in other words, science as it is has gifted us all these things and many more and even though science is merely a tool by which we understand ourselves and our place within the eternal, it still is a powerful friend when we need it . You cannot cherry pick. You either trust science, when it supports the life you lead or you don't.
If there are aliens constantly travelling from their homeworld to ours, and I am agnostic on that subject, then they have to be related to us. This is pure logic. The physical similarities between aliens and humans are strikingly remarkable given the so-called evidence supplied. However, we cannot proselytise the possibility that one species has mutated or even evolved from another without discounting DNA. Now Deoxyribonucleic acid is a molecule. It carries genetic instructions used in the growth, development, functioning and reproduction of all known living organisms, but also a great many viruses. There is a combination of acids, proteins and lipids mixed with some complicated carbohydrates that form the four major types of macromolecules that are vital for all known forms of life. This fact is empirical, unassailable. This suggests that aliens and humans, related for sure, have not originated from a single source unless that source is Earth for human life is linked via DNA to the planet Earth, Which can only mean that aliens are human or that somewhere in deepest space is a planet exactly, or very nearly exactly, the same as our mother world.
If this is the case then the reason for the visits cannot be aggressive. What would be the point? They want to enslave us? Why? For what purpose? It cannot be their desire to mate with us. Millions of years ago we humans could have reproduced with Chimpanzee's or what was the forerunner of the chimpanzee. We couldn't now. Is it being suggested that aliens like a bit of bestiality? That is exactly what a man having sex with a chimp would be. Or could it not be that the aliens, recognising our kinship, simply want to observe and to study a species so alike their own yet woefully less technological, far more primitive, to monitor the growth of humankind? This sounds far more likely.