There is an emerging consensus regarding important parts of the story of how modern humans populated the world during the Paleolithic era. I recap the facts as I understand them, with no more specificity than widely held scientific views seem to support, in this post.
Modern Humans Evolve In Africa from Great Apes; Archaic Hominins Emerge From Africa
1. In Africa, modern humans evolved on a branch of the great apes whose closest living relatives are the chimpanzee and the bonobo. Intermediate hominins evolved from these great apes, probably including species that were evolutionary "dead ends" including (to the extent that they did not admix with modern humans), non-African Homo Erectus and Neanderthals. There are something like half a dozen proposed African intermediate archaic hominin species in at least two or three genus level classifications although the precise species classification of individual specimens, and the relationships of these specicies to each other are disputed since the evidence is often fragmentary and isolated.
2. All Eurasian hominins have ultimate evolutionary origins from the same proto-hominin ancestor in Africa as that of modern humans who was most closely related to the chimpanzee and the bonobo.
3. Modern humans were not the first species of hominin to leave Africa. Homo Erectus was the first hominin to leave Africa and first appeared out of Africa around two million years ago, give or take a couple of hundred thousand years.
4. Sometime around 250,000 years ago (with fairly wide error bars on that date), the most recent common ancestor of all modern humans evolved somewhere in Africa. Estimates of this date from archaeological evidence and different methods of mutation rate driven genetic estimations are not identical, but are consistent with each other to margins of error on the order of plus or minus 50,000 year.
5. Modern humans evolved after Homo Erectus and Neanderthals evolved. Modern humans are not directly ancestral, except due to two or more introgressions of archaic hominin DNA none of which is common to all modern humans, to Homo Erectus, to Neanderthals, to Denisovians (whatever species they may belong to), to the Homo Florensis.
6. At the time the first modern humans left Africa there had been Homo Erectus and Neanderthals in Eurasia before them, and there may have been other or intermediate or hybrid archaic hominin species as well. No hominins, however, had ever reached Australia or the Americas or Antactica or Oceania, before moder humans did.
Modern Humans Leave Africa
7. Modern human initially migrated out of Africa via either Yemen or the Sinai Pennisula or both, Modern human did not make their original emergence out of Africa via Spain and not via direct ocean or sea travel to anywhere else. It follows, of course, there there were modern humans in Southwest Asia before there were modern humans anywhere else outside of Africa. At the time that modern humans migrated out of Africa, Neanderthals were present in the Levant and Europe.
8. The oldest hominin remains widely accepted as being anatomically modern human outside Europe are at least 100,000 years ago. There are stone tools in non-coastal Arabia from an era when that was not a desert that show strong commonalities with stone tools from the Nile Basin at the same time from thousands of years earlier. There appear to be a wealth of stone tools, probably but not definitively associated with modern humans (they could have been Neanderthal tools) in this interior Arabian basin dating from sometime in the MIS 5 interglacial period which began ca. 130,000 years ago. There is no archaeological evidence for modern humans outside African more than 130,000 years ago and there is nothing in mutation rate based genetic dates that demands an out of Africa date that old.
9. There is dispute between moderns in which (1) there was an Out of Africa that failed wave starting sometime around 130,000 to 100,000 years ago in Southwest Asia, possibly without penetrating much further, and ending 75,000 years ago, followed by an Out of Africa wave that took hold 60,000 to 50,000 years ago, and in which (2) the first Out of Africa wave ca. 130,000 to 100,000 years ago was successful and is in continuity with all subsequent non-African populations with the archaeological evidence of modern humans gap from about 100,000 to 75,000 years ago being due either to poor preservation of modern human artifacts and remains, or to relocation of proto-Eurasians to non-Levantine refugium(s) including potentially the Nile Basin, the Horn of Africa, interior Arabia, the Persian Gulf, Iran, and South Asia that remain largely undiscovered at this time. Neither theory is inherently inconsistent with minor subsequent Out of Africa waves of migration, although this possibility doesn't receive wide attention.
10. All modern humans with recent origins outside of Africa have a low and fairly consistent percentage of Neanderthal admixture in their autosomal genome. Neanderthal admixture is absent from modern humans with no ancestors outside of Africa. The specific Neanderthal genes seen in East Eurasians and the specific Neanderthal genes seen in West Eurasians overlap only slightly due to some combination of parallel and similar, but separate, admixture events in a proto-West Eurasian and a proto-East Eurasian population, schism of an admixed proto-Eurasian population before admixed Neanderthal genes had reached fixation, unfitness based selective loss of Neanderthal genes that had negative functionality relative to modern human variants, and loss of selectively neutral Neanderthal genes due to random genetic drift experienced in serial founder effects particularly at population bottleneck moments for West Eurasians and East Eurasians respectively.
11. The fact that the specific Neanderthal genes found in West Eurasian and East Eurasian populations differ materially from each other also supports a conclusion that the population genetic isolation of West Eurasians from East Eurasians during the Paleolithic era was almost complete.
12. A small subset of Neanderthal admixed genes are found in modern human populations at frequencies that strongly suggest some sort of fitness enhancing selective effect that have caused them to be present at frequencies not consistent with selectively neutral random chance in their inheritance.
13. No modern human men have Y-DNA haplogroups attributable to Neanderthals or any archaic hominins (of course, Y-DNA is uniparentally inherited patrilineally). All modern human men with recent origins outside of Africa have Y-DNA haplogroups have root in either the most recent common ancestor of macro-haplogroup DE, or the most recent common ancestor of Y-DNA macro-haplogroup CF. Both of these Y-DNA macro-haplogroups are rooted in a single modern human lineage, of which Y-DNA haplogroups A and B, which are private to Africa, are more basal.
14. No modern humans have mtDNA haplogroups (a matrilineally inherited uniparental genetic marker) attributable to Neanderthals or any archaic hominins. All modern human men with recent origins outside of Africa have mtDNA haplogroups have root in mtDNA haplogroup M or mtDNA haplogroup N, both of which are offshoots of mtDNA haplogroup L3. The roots of mtDNA haplogroup L3 are African. The roots of both mtDNA haplogroup L3 and all other mtDNA sub-haplogroups of mtDNA haplogroup L, many of which are more basal than the root of the L3 haplogroup, arise from a single mtDNA most recent common ancestor ("mitchondrial eve"), and all or almost all of the sub-haplogroups of mtDNA haplogroup L after private to Africa or predominantly African in their distribution. While mtDNA subhaplogroup M1 is found in Africa (predominantly Northern and Eastern Africa), it is widely believed to have back migrated to Africa from Southwest Asia sometime long after the Out of Africa event, probably in the Upper Paleolithic era. Other non-haplogroup L mtDNA haplogroups are believed to be more recent back migrations to Africa and are rare in non-Afro-Asiatic linguistic populations with a handful of exceptions near the Afro-Asiatic linguistic boundary or attributable to historic era migrations.
15. There is no other genetic indication of archaic hominin admixture with modern humans outside Africa at this time.
16. There are some indications from African autosomal genetics that there may have been archaic admixture, possibly two separate events in different places with different archaic hominin species as recently as 18,000 years ago, with the ancestors of relict hunter-gatherer populations in Africa, Southern African Khoisan peoples and Pygmy peoples. Genetic evidence suggest that these "Paleoafrican" populations have the most basal break from other modern human populations in tree-like modern human genetic phylogenies. The most recent common ancestor of other African populations, including the African populations from which all non-Africans are genetically derived, is more recent than the Paleoafrican branch's split from other Africans. There is no evidence at this time of archaic hominin admixture (i.e. admixture with archaic hominin who has speciated from modern humans long before the species was established who were not directly ancestral to modern humans) with the ancestors of African populations other than Paleoafricans that is not derivative of Eurasian Neanderthal admixture.
Modern Humans Arrive In India and Expand Into The Rest Of Asia From There.
17. There were modern humans in India before there were modern humans in Europe.
18. The earliest dates suggested for modern humans remains in Asia to the east of India come from around 100,000 years ago in Southern China, although their classification as archaic v. modern human is hotly disputed, the finds are outliers, and the dates are subject to question due to their outlier status and major corrections to other Asian archaeological dates in the last half century. Likewise the latest widely accepted dates for archaic, non-Neanderthal hominins in Asia to the south of Siberia and to the east of Pakistan are about 100,000 years ago.
19. The latest possible date for modern humans in Asia is 45,000 years ago, when there is definitive Papuan and Australian evidence, plus at least a few thousand years from migration from India to Papua New Guinea and Australia. The archaeological record is too thin to support any particular date of modern human arrival in Asia beyond India in the time period from 45,000 to 100,000 years ago, and the mutation rate based genetic estimates are not definitive enough to clearly resolve the question.
20. The extent to which lithic technological cultures can be definitively associated with particular hominin species is hotly disputed and distinguishing between stone tools found in sites where they are definitively associated with modern humans in the Mesolithic (i.e. basically after modern humans evolve, but before modern humans arrive in Europe) from stone tools found in sites that are definitively associated with Neanderthals, without the remains to guide that determination, is a subtle art. Only in subsequent Upper Paloelithic era do the distinctions between modern human tool artifacts and Neanderthal artifacts become more obvious. In earlier periods when modern humans coexisted with Neanderthals in the Levant, the distinctions are far less clear.
21. Similarly, while there are clear distinctions between more primative Asian archaic hominin stone tools first used by Homo Erectus and the first distinctive, pre-modern human era stone tools of Neanderthals in Europe, the extent to which different tool technologiess strictly correspond to a Homo Erectus v. Neanderthal species distinction in liminal areas like South Asia, when not found in association with hominin remains, is hotly disputed.
22. The modern humans who initially arrived in Southeast Asia, Indonesia, Papua New Guinea, Australia, East Asia, Tibet, Oceania and Japan were all descendants of modern humans who traversed India (i.e. they arrived via a "Southern route"). It follows, of course, that there were modern humans in India before there were modern humans in any of these other places. The modern humans who first peopled all these places with modern humans did not have ancestors who made their way into Asia from Europe or Central Asia or Siberia. The population genetic evidence to support this proposition from Y-DNA, mtDNA, the details of Neanderthal admixture, Denisovian admixture, and autosomal genetics is overwhelming and consistent on this point.
23. The Toba volcanic erruption, the largest in the pre-historic out of Africa era, took place approximately 75,000 years ago in Indonesia. The direct effects of the Toba erruption (e.g. notable deposits of ash) ranged mostly from the Toba volcano in Indonesia to India (with magma itself probably only affecting the unfortunate residents of the island itself). What are currently the jungles of Burma would have been particularly hard hit.
24. It isn't entirely clear what kind of global climatic impact the erruption may have had. The most recent climate models have tended to downplay the extent to which the Toba erruption could have wiped out modern humans in the Levant (in a failed out of Africa scenario) or wiped out Homo Erectus in Asia very far to the east of the volcano. The Toba erruption roughly coincides with the beginning MIS 4, the cooler Paleoclimate period following the interglacial in MIS 5.
25. There were probably modern humans in India both before and after the Toba erruption as indicated by lithic tools on both sides of deposits of Toba ash there that are in continuity with later confirmed modern human populations there.
26. The biogeographic Wallace Line that has mainland Southeast Asia and most of Southeast Asia and the Phillipines on one side, and Papua New Guinea and Australia on the other, has never been crossed by a land bridge at any time that hominins have been present outside of Africa (i.e. in the last two million years).
27. Papuans and aboriginal Australians have autosomal genetic profile that show substantial admixture (about 8%), in additional to older East Eurasian typical Neanderthal admixture of about 2.5%, with archaic hominins substantially similar to ancient DNA extracted from bones in a Siberian cave at Denisovia. There may have some independent traces of Denisovian admixture in Phillipino negrito populations, but all other Denisovian admixture in the world is traceable to these sources.
28. The arrival of modern humans in Australia triggered by some means a mass megafauna extinction there, and took place at a time when Papua New Guinea, Australia and Tasmania were a single continental mass. Modern humans were present in Papua New Guinea and Australia around 45,000 years ago and not too many thousands of years before then.
29. The arrival of modern humans in Australia and Papua New Guinea, as I understand it, took place at a time when Indonesia up to the Wallace line has not yet been connected to mainland Asia as it was during the last glacial maximum when sea levels were lower. Indonesia was (and the Philippines were), as it is today, an island chain divided by shallow water ocean straights around 45,000 years ago.
30. It is not clear if there were land bridge moments in Indonesia after the arrival of Homo Erectus ca. 1,900,000 years ago (Java man) and before the Last Glacial maximum, although it doesn't seem unreasonable to think that there might have been such time periods.
31. Homo Florensis appears to have been in Flores from ca. 100,000 years ago until about 18,000 years ago and to have co-existed with modern humans for tens of thousands of years on the island with is on Australia's side of the Wallace line.
32. The population genetics of Asia indicate that the Paleolithic demographic history of the region is probably more complex than a single wave of pre-Neolithic migration, and is complicated by major impacts from Neolithic era migrations.
33. Modern humans arrived in Japan around 30,000 year ago and in Tibet sometime before the last glacial maximum.
34. There was a megafauna extinction in Siberia around 30,000 years ago.
Modern Humans Arrive In Europe and the Americas, Archaic Hominins Go Extinct
35. Modern human migrated to Europe around 45,000 years ago, give or take a few thousand years and were found throughout Europe within a few thousand years.
36. The arrival of modern humans in Europe coincided with a period of major volcanic erruptions in Southern Europe and at least some period of volcanic ash induced climatic cooling.
37. Neanderthals and Siberian Denisovians were extinct by about 28,000 years ago, well before the Last Glacial maximum and well before the founding population of the Americas in Beringia was established.
38. The last glacial maximum was about 20,000 years ago. Most of Northern Europe and Siberiia was underneat glaciers at this point in time. Sea levels were lower then than at another other time in the Out of Africa era giving rise to the Berginian land bridge and the connected Sahul land mass.
39. After modern humans arrived in Australia, ocean straights divided Papua New Guinea and Tasmania from its land mass.
40. After Tasmania became separated from Australia, a small number of dingos were introduced in Australia from Southeast Asia (presumably via Indonesia) and this triggered a secondary wave of marsupial animal extinction in Australia.
41. Homo Florensis (in Flores, Indonesia) and any archaic hominin species in Africa were extinct by about 18,000 years ago, leaving modern humans as the sole surviving species of hominins on Earth.
42. The founding hominin population of the Americas consisted entirely of modern humans who (with the possible exception of a handful of individual on the Atlantic side of the continent divide who had no material long term demographic or ecological impact) arrived there via a Beringian land bridge within plus or minus a few thousand years of the Last Glacial Maximum, and made their way into the rest of North America and South America sometime within a few thousand years after the Last Glacial Maximum. It took no more than a few thousand years once this passage has opened for modern humans to make it from Beringia to the farthest point in South America. The arrival of modern humans in the Americas triggered, by some means, a mass megafauna extinction in the Americas.
43. The founding modern human population of the Americas predominantly derive from peoples who ancestestors made their way to Beringia via the Southern route through India, although there may have been a minor component of the founding population of the Americans (exemplified in the mtDNA haplogroup X found in indigeneous Americans) that made its way to Beringia instead from Southwest Asia via a Northern route through Siberia. It isn't entirely clear if this Northern route component was part of the initial founding population wave of a later wave of migration into the Americas.
44. There are substantial population isolation genetically between South American and North American populations in the pre-Columbian era.
45. The sudden cooling period of the Younger Dryas, following a post-last glacial maximum warming period, was triggered by an extraterrestrial impact about 16,000 years ago. We are now in an interglacial climate period known of the Holocene which started around 10,000 years ago.
46. There appear to have been major population migrations in the time period after the last glacial maximum and before the Neolithic revolution in the Mediterranean basin, as well as a repopulation of Northern Eurasian latitudes.
47. The Na-Dene peoples of North America derive genetically, in part, at least, from a wave of migration sometime after the founding population of North America and sometime before the Inuit people arrived. The Na-Dene migrated to the American Southwest around 1000 years ago.
48. The modern Inuit population of North America derives in substantial part from Siberian ancestors who were not part of the founding population of North America and instead arrived as part of the Thule migration wave, an archaeologically well dated migration wave after the Dorset people, during the historic era (although pre-Columbian).
49. A historically attested and archaelogically confirmed group of Vikings with roots in Europe associated with Lief Erikson arrived in North America about 1000 years ago in coastal Canada, but the colony quickly collapsed, had no major genetic or ecological impact on North America, and is not clearly attested in any surviving indignenous American folklore.
50. The arrival of Europeans in the Americans with Columbus in 1492 had dramatic effects, mostly on the Americas but also on the rest of the world due to impact flow from the Americas.