Mating systems are not preserved in fossils, and mating system (monogamy, polyandry, polygyny, promiscuity etc.) are adaptive. That means it can change. Most primates, including lesser apes like the gibbon, are monogamous. Of course even monogamous species will cheat. Yet our closest relatives have different mating...
Best answer: Mating systems are not preserved in fossils, and mating system (monogamy, polyandry, polygyny, promiscuity etc.) are adaptive. That means it can change. Most primates, including lesser apes like the gibbon, are monogamous. Of course even monogamous species will cheat. Yet our closest relatives have different mating systems. orangutans do not form pair bonds. Neither do chimps or bonobos. Gorillas are polygynous, with a single dominant male forming a family with multiple females. The dominant or silverback male is the only one allowed to have sex with his females, but of course cheating also happens. Chimpanzee females however may have sex with multiple males within the same troop. Bonobos have sex daily and frequently and it is a free for all. In fact, bonobos will have sex with juveniles.
What about humans? Humans are mostly monogamous. However, there are some societies that have permitted plygyny and a small number practice polyandry (one woman having several husbands during her lifetime) although usually one at a time instead of several at the same time. Humans evolved as hunter/gatherers 150,000-200,000 years ago, and we evolved from an ape in Africa beginning about 5 million years ago. Since written records date back to no more than about 5,000 years or less for most human civilizations, we have no idea how humans and our ancestors mated for most of that 5 million years.
Nevertheless, we can compare humans with our closest relatives and try to figure out. Anatomically, humans are different from all other primates in having the largest penis (bigger than those of the gorilla and chimp) of any primate. We are also the only primate to lack a penile bone. Humans also differ from other primates in having small canine teeth. Indeed, Ardipithecus, which is possibly our ancestor that lived 4.5 million years ago, has reduced canine teeth. The canine teeth and the large penis inform us how our ancestors may have mated. Scientists interpret the reduced canine teeth as evidence of reduced male-male combat. Male-male combat usually occur in species that compete for the right to mate. If males do not fight over females, then individuals with reduced canines are not disadvantaged and they have a good chance of mating. If males do not fight over females for the right to mate, they may in fact cooperate with each other by forming troops in order to survive. Since humans evolved in the dangerous African savanna, forming troops with multiple males would indeed help our ancestors survive. Alone, a single human is no match for lions, leopards and hyenas. However, if human ancestors formed troops, they can beat back predators like lions, as Masai warriors do by surrounding a lion and spearing it to death.
If human males formed troops and they do not fight each other for the right to mate, that would mean that humans could not have been monogamous when we first evolved. Indeed chimps also form troops but for a different purpose. They form troops with multiple males to defend territories inside forests. Chimps too do not beat up each other over the right to mate but instead they have evolved a longer penis (than the gorilla) with huge testicles, so they instead engage in a war of the penis. It means only chimps with a long penis and the ability to store a lot of sperms in their testicles managed to put enough sperms inside a female chimp to have a good chance of having one of his sperms fertilize an egg even though a female chimp may have mated with another male earlier. The long penis put sperms closer to the uterus so that they have a smaller distance to swim. The human penis is longer than the chimp penis, and therefore we can put sperm close to the uterus as well. In fact scientists point out that the human penis is shaped in such a way that it can shovel sperms that are already inside the female out of her body. Female humans also have the unique ability to have multiple orgasms, and that allows them to have sex with many males and still enjoy it. She may want to do that because if she mates with multiple males, they may all think that they may be the father and they won't harm her child when it is born. Therefore it benefits the female to have sex with multiple males too.
If human anatomy and physiology suggest that both male and female humans originally evolved to have multiple sex partners, then when did humans (or our ancestors) stopped being promiscuous and started demanding monogamy? We may not be able to know for sure, but since most animals that are monogamous or polygynous in nature provide male parental care, then it may be the same for humans. Males want to make sure that their mates do not have sex with other males if they are responsible for raising young. Cuckoldry is costly to males, as males that raise the young of other males will end up having fewer young that are his, and so their genes would be lost if they do not demand and ensure monogamy. So, when did it happen that males were able to live without forming troops and they needed to provide parental care?
The likely answer is that agriculture enabled human males to stop forming multi-male troops. They could settle down and raise a family of their own, without help from other males. That also mean that they are now solely responsible for raising their own children, since no other males will help them out by bringing back food to share with the entire troops. Agriculture was invented only about 10,000 years ago. That means it was a rather recent change if humans changed from being promiscuous to being monogamous. If the change is so recent, it is little wonder that some societies still practice or tolerate polygamy and even promiscuity. Indeed some humans still live in hunter/gatherer groups with multiple males. Indeed many of these types of groups still practice open marriages, and the men allow their wives to have extramarital sex.
As to your point about scientists not knowing everything, that is of course correct. Scientists rely on facts, and sometimes it is not so easy to gather facts. Indeed humans are not animals that are born with instincts so that they all act alike. We can act differently as individuals or as different populations. Our behavior and culture can also change if our environment changes. Indeed native Americans were traditionally promiscuous (both men and women are allowed to have sex with any other freely), as they live in tribes with multiple males forming troops to defend themselves. In contrast, monogamy is practiced almost universally in agricultural and industrial societies in which males are responsible for parental care.