Marine isotope stages (MIS) are periods that alternate hotter and colder climate. They have been identified by measuring the content of oxygen isotope 18O in ice cores.
They are numbered, and the MIS with even numbers represent glacial periods (and therefore lower sea levels as the ice packs have taken water out of the oceans), and the odd numbered stages represent warm interglacial periods (like the current one), with higher sea levels.
So for humans to cross Beringia and reach America, they'd have to do so during a cold even numbered MIS.
This is the list of cold MIS during the past million years:
- MIS 2 – 29 ky
- MIS 4 – 71 ky
- MIS 6 – 191 ky
- MIS 8 – 300 ky
- MIS 10 – 374 ky
- MIS 12 – 478 ky
- MIS 14 – 563 ky
- MIS 16 – 676 ky
- MIS 18 – 761 ky
- MIS 20 – 814 ky
So why did our human ancestors cross during the last possible stage? Why didn't Neanderthals do so during MIS 10? or Homo erectus during MIS 2O?
The following map, adapted from Nature shows that there were humans in Asia that could have crossed into America from MIS 8 onwards, and this does not contemplate H. erectus.
They lived in temperate to cold locations, why not follow the megafauna into Siberia and North America?
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Hello Austin. Totally agree with the essence of your post.
ReplyDeleteDuring the marine isotopic stages you have listed, it was highly probable the existence of “windows” of time of variable duration that could have perfectly complied with the two needed conditions for a fully terrestrial path from Asia to America to be feasible;
- a) Sea level at -50/-55m, or lower
- b) “Moderate” (albeit cold…) climatic conditions in Northern America, characterized by extended areas virtually free from ice.
These conditions could have taken place at interstadials, periods within which not so rigorous climate is expected to have prevailed during several Ka, or during the “terminations” of the mentioned MIS ( high sloped transitional periods between the ending of peak glacial conditions and the onset of the warmer ones of the subsequent interglacial) that are highly suspected to have presented at least some Ka of consistency with the above mentioned requirements…At this respect, it would be interesting to remark that any of the climatic cycles occurred since 1 Ma ago, has had a well defined “termination” and not less than one or two interstadial periods…
In answer to your open questions, I am also inclined to speculate that whether motivated by their needs of food (indeed, a very strong reason…), or even by their natural curiosity to try to discover what was beyond their lands (an aspect that is 100% inherent to our species), the Asian potential immigrants you have mentioned;… H. erectus in various stages of his evolution, as well as the hybrid species that evolved in East Asia around MIS8, bearing a variety of traits ranging from Neanderthal ones (some of them probably Denisovan??) to others that were closer to a modern H. sapiens, … have had no insurmountable impediments to make their way into America.
Furthermore, the “windows” they took advantage of, could have been considerably broader in time if we consider the not so reckless possibility that these Homo could have been capable of transposing relatively short distances of sea, by means of some sort of rudimentary navigation… as this could have allowed them, for example, to cross Behring Strait with higher sea levels (-35/-40m, or even higher) by “jumping” into islands within sight…with the benefit of an inherently warmer climate at their entry to America.
Best regards
Marcelo