The last common ancestor (LCA) between humans and their closest genetic relatives, Chimpanzees and Bonobos, is found roughly a quarter million generations ago, estimated by Richard Dawkins to occur between 5 and 7 million years ago. The first species of the genus Homo to appear distinct from other apes is found about 2.5 million years ago. The first Homo sapiens appeared about 160,000 years ago (and using other assumptions a distinctly “modern human” with the same faculties we humans share today could be found walking about 50,000 years ago). “Our” record of civilization extends back to three isolated language groups, the Indus (Mehrgarh and Lahore city-states), the Elam (Susa city-state), and the Sumer (Ur city-state), which all traded one with another beginning only about 250 generations ago. The most recent common ancestor (MRCA) of humans alive today, which by fecundity crowded out many other humans that lived with it, appeared 2000 to 4000 years ago, which averages to about the past 100 human generations.
During the last 1500 years and before the advent of birth control, the average period between generations has been 30 years. This is known from accurate genealogical records. It is calculated by dividing the number of generations into the years elapsed. I’m sorry we don’t have a reference to publish for this. It is borne out by an author’s own genealogical work with numerous lines over 10 generations and the lineage of an acquaintance who was descended from Mohammed (his was 29.5). Also, many other genealogical results fall close to 30 years per generation. To use another tiny sample, from my own Grandpa’s birth in 1905 to my birth in 1962, two new generations consumed 28.5 years each. If I were somehow late in life to produce a child in 2018, let’s see, that would throw off my family’s average to 36 years each for three generations. But you can see how, over many generations, the years between generations average about 30.
Using 30 years per generation, Homo appeared distinct 83,000 generations ago and Homo sapiens appeared 5,300 generations ago, with fully modern humans appearing just 1,666 generations ago, and the roots of western civilization (from the Euphrates to the Indus rivers) developing 250 human generations ago.
I believe it remains an open question as to whether other stable periods of suitable climate produced other unknown, and possibly technological tropical human civilizations during those long years that ramped up the last ice age or during its stable maximum, or even during the previous warm period and prior ice age that preceded it. If such occurred, they apparently had better energy alternatives than to exploit fossil carbon fuels, or perhaps found them too inaccessible from civilization centers at the time, all of which would have been located in the tropics of a 5 degree C cooler Earth near their earlier, lower shorelines than the one humans experienced during the Holocene age. To search for possible ice age civilization(s), we would need to look near major river mouths to evidence buried beneath the sea on the order of 10,000 years ago located at a current depth of around 400 feet. It is worth pointing out that a millennium from now our own descendants will have to search for the remains of our current civilization’s great centers at rest beneath up to 212 feet of seawater that increasingly inundated coastal communities until the last Antarctic icecap disappeared and climate stabilized at a global temperature five or six degrees C higher than today’s. The long dinosaur ages occurred during that kind of climate, when no sea ice or glaciers were able to form for hundreds of millions of years. However, the amount of free oxygen in the atmosphere from the previously overgrown Carboniferous age was higher back then helping to make such great and terrible animals possible.
But what of commonly read records of ancient civilizations? Hebrew biblical records seem to owe much to Babylon’s preservation of a Sumerian Kings List. The Sumerian Kings List appears to reach back to the Rissian Stage glacial, a date contemporary with some of the earliest Indian paleoart and ostrich shell disc beads from Lake Fezzen in today’s Libya. The earliest record in the Sumerian Kings List is that of Alulim of Eridug, noting that “after the kingship descended from heaven” (perhaps meaning came down from the mountaintop refuge used by survivors of the prior ice age meltdown?) “it was in Eridug.” Though it is dismissed as mythical by today’s scientists, a literal reading places the historical era of Eridug as the ruling Sumerian city according to the Kings List extended from about 263,792 years ago to 198,992 years ago, a period of around 64,000 years. Did Alulim live and rule for 64,000 years? Hardly possible, but just as we remember the early years of the United States from 1776 to 1812 to be dominated by its first President, war hero George Washington, perhaps the people of Sumer remembered their earliest history to have been dominated by Alulim in Eridug. This may have been a dynastic reign, but I doubt that too, knowing how much world history can happen in only the short 7,500 years within which “our” civilization incubated from agriculture to driving to the moon and Mars. Instead I would propose that the earliest events in a history become compressed and associated with the most memorable story from those times. For instance, regarding the 239-year history of the United States, I would recall that its early days are remembered for George Washington winning the American Revolution up to the time of the 1812 War with Great Britain, and then Abraham Lincoln fighting an American Civil War that filled the 19th Century, and then the World Wars’ Roosevelt’s and the post-war Kennedys’ Camelot-like American (20th) Century when a fearfully nuclear-armed USA dominated its world. Then its detailed modern history ensued with a long War in Iraq and constant fears about terrorism and a slow decline of its empiric fortunes and power. Such a synoptic view of history would seem more common the farther back the memory ranges.