There are many theories that are put forward to explain stories about giants. These include that there actually was a race of giants, that the stories are recalling a now extinct subspecies of hominid. I have also seen stories about giants justified by the idea that people had found mammoth skulls and then invented the idea of giants - thus creating the stories. I have also heard people put forward the idea that stories about giants are evidence of ancient alien visits.

These are theories (and I'm sure there are many others) to explain why diverse people have stories about giants in their oral traditions. I suspect that the many removed comments to this posting reflects how people are intrigued by these theories and put them forward in response to this sort of question.

The question here is whether these are reasonable theories. Embracing these sorts of ideas has become an aspect of modern folk belief. The core folk belief at play here is that "all legends have an element of truth embedded in them." There is no evidence that this is true; this is part of modern folklore. Theories about the origins of giant folklore may be fun to think about, but there is absolutely no evidence to reinforce them.

A first point: stories were not likely that widespread; those expressed in cultures speaking Indo-European languages likely came from a single common ancestor rather than from unique, diverse and widespread traditions. Non-Indo-European-speaking cultures occasionally have stories about giants, but they are not necessarily as widespread as is occasionally asserted.

That aside, many (but not necessarily most) pre-modern folklore collections describe a giant or more than one giant playing a role in the creation of an aspect(s) of the world. These are what is known as etiological legends - narratives told to be believed that explained the origin of something. Stories about giants typically involve times in a remote past or a remote location: people would not describe actually seeing giants in their own time - unlike elves/fairies/trolls/hiddenfolk/etc., which people believed it was possible to encounter. It was impossible for people to imagine that a giant was hiding nearby because they would be easily found out; it was easy for people to imagine elves hiding nearby.

The premodern process related to giants typically followed the following course: look at that enormous stone in that field; how did it get there? it would have taken the strength of someone who was enormous; a giant must have thrown it there, but since no one remembers that happening, it must have happened long ago. The folk believed that there were once giants, and so it was easy to see the consequences of their existence in the world around them.

Because traditional folk belief relegated giants to an ancient time, modern folk belief follows the following course: many ancient peoples had stories about giants; all legends have an element of truth within them; the stories must be linked by something that was real (because all legends must have something real behind them); therefore, there must have been a race of giant humans that may have been a subspecies of hominids or aliens or ... fill in the blank with speculation.

Those lines of speculation are not theories resting on facts. Diverse people had stories about giants because some of these stories diffused with Indo-European languages. These and other stories reflect a natural process of people attempting to grapple with the origin of the remarkable world around them.

Oh there are theories...But here I'm going to give a radically different answer than itsallfolklore's because I think there is a pretty good reason for why so many societies have beliefs about giants, and that's because there were giant (animals) that existed on earth. Ancient people found these huge yet-recognizable bones (which had turned to stone in primordial time) and attempted to historicize these strange objects; doing a form of ancient natural science.

So, aside from giant animals, there are some ancient societies who believed in giant humans existing in the primordial past; and of course this was not the case. So how does a story which definitely did not happen become a relatively common trope? Well this is where theory comes into play, because some researchers explain this phenomenon as an evolutionary mechanism. But I think that is too simple, because stories are not told to convey positive reproductive values, but are told because they are captivating, resonant, believable, were told by an authority, seem to be true, or are true / are history. I've written about a few other paleolithic stories and why exactly are they continued today? Well sometimes a story is still told simply because it started, and it was compelling enough to have been retold. Stories about giants were and are told because they are thought to be true by the storytellers - whether it's an oral history about primordial time depicted in rock art, or a retelling of the sacred Book of Enoch in an east African church/synagogue. We do not know the first person to tell such a story, but as stories about giants and little people are seen around the world both appear to be either 1) paleolithic stories that have been re-told for thousands of years, or are 2) simple enough tropes that they happen to be reinvented multiple times in recorded history. While I'd argue that their general form is a paleolithic story, u/itsallfolklore has argued for the second position such as in this post about the idea. Imo the occasional natural occurrence of dwarfism and gigantism would've reinforced these stories with verifiable examples and helped keep the stories alive/relevant.

A list of giants... - Adrienne Mayor has a wonderful book pointing out how Greeks and Romans found giant fossilized bones and presumed them to be local giant heroes who existed in the primordial time. And similar notions were hit upon by Mesoamericans (at least Tlaxcaltecans) who said the same thing about giant fossil bones at the period of Euro contact ca. 1500 CE. Other peoples in the Americas also had stories of primordial giants, such as the Nahullo of the Chahta (Choctaw) spoken of in the 19th century. David Cusick, a Skaru:re? (Tuscarora) and Haudenosaunee (Iroquois) man wrote in the 1820's that there was a "tribe" of primordial giants called Ronnongwetowanca. There are many other stories, ranging from Algonquin, Iroquoian, Caddoan, Muscogean, Salishan, Yakonan, Athabaskan, and Uto-Aztecan speaking peoples. A:shiwi (Zuni) people said/say that after creation, the Twins roamed the earth and killed monsters and giants.

While hand prints are relatively common in world-wide rock art, there is a larger-than-life-sized human hand print with its palm decorated as a spiral found along the Riacho Palmeira river in Piracuruca, Piaui state, Brazil. Giant petrosomatoglyphs (body print rock art) are also seen at the late bronze age to early iron age Canaanite temple at Ain Dara (in what is now northern Syria), which featured 90cm / 3' long giant foot prints carved into the floor of the temple. These must have been a part of its worship ceremonies...how exactly, we're not sure; but a good guess would be in relation to stories about primordial giants.

20th century Tassilian Tuaregs have a story of a primordial giant culture hero named Amerolqis who "greatly surpassed men [in height]" and was/is described as wullen wullen (big, abundant). So much so that when he impregnated human women they would burst (because the infant was giant)...so he eventually impregnated elephants. Alkhussayna, a Kel Nag Tuareg blacksmith said in 1975 that it only took 3 days after becoming pregnant for them to explode, brutal.

It's fascinating that the story is set in the primordial past, because the inclusion of elephants is another set piece in that narrative; while some Tuareg were still able to hunt elephants in the early 20th century, many have not seen elephants and some don't know of their existence. The story is certainly old and widespread, as versions of it (with the main character being simply a human culture hero) are shared by other Tuareg groups as well as the southern Chadian Nilotic speaking Ngambay people. Jean-Loic Le Quellec thinks this story is related to late neolithic period (ca. 3000-1000 BCE) central Saharan rock art in which larger-than-human-sized therianthropes (animal-humans) are shown having sex with women (in the Acacus) and elephants (in the Messak). Perhaps this is related to Pliny the Elder's comment that King Juba II of Numidia and Mauritania said that elephants could mate with humans.

Other Tuareg stories say their ancestors are giants called Ijobbaren, Ixazamen, or Isebaten, and that they lived "in a time when the stone was soft" and thus are related to the explanation of rock art. Local stories from Filingue (southwest Niger) include giants with big ears and feet who can eat a giraffe as a single meal. Maghrebi oral stories include giants with magical powers who have a one form in which they are half human and half animal, and are described as being covered in hair and having powerful jaws and tusks.

Tuareg also named the Jabbaren area of Tassili n'Ajjer in the central Sahara after the giant figures seen in its mid-Holocene forager period rock art (called the Round Head period/style, ca. 6000 BCE) such as "The Great God of Sefar" who is about 6m / 20' tall. Other "giants" are seen at Karkur Talh North site 52 where an "early Round Head" giant is seen next to a much smaller giraffe. Contemporary with the Round Head period is Korossom North site 12 in Tibesti, Chad, which shows a large and very unnatural humanoid in red next to a smaller human in yellow.

Late neolithic period engraved rock art by pastoralists at the Fuchs panel, Niola Doa plain, Ennedi, Chad, show a usual scene of cattle and a human but to the left of the scene is a miniature human (about the height of the cow's knee) who stands looking up at this giant beast. It's possible the mini-figure was a later addition to the more standard scene, or perhaps this represents primordial giants who also had primordial giant cows. Hugely larger-than-life and wildly unnatural cows are also seen in Saharan rock art, so this is a possibility. At Karkur Talh North site 31 other late neolithic rock art shows a group of archers surrounding and shooting a significantly larger humanoid only armed with a melee weapon. And at another late neolithic site of Karnasahi 10 in Tibesti, Chad, shows a huge (1m long) seated human surrounded by a more normal scene of grazing cattle which are much smaller.

Other larger-than-life human figures are seen in the Proto-San rock art of southern Africa, such as the "Hunched Giant of Inanke." More recent Tsatsane San art at Ha Lipaphang site in Lesotho show small black figures surrounding and shooting arrows at a few much larger white figures.

The Bradshaw Foundation also has a picture on its website of Australian rock art showing a panel of small "Tassel Bradshaw" figures (late pleistocene) with two huge and noticeably differently-styled figures behind them in the pre-Bradshaw "Naturalistic" period/style.

And of course, anyone who's heard about ancient giants has heard about the Hebrew tradition of nephilim; which perhaps weren't originally giants but certainly had become such by the Hellenistic/Hasmonean period.