By land: until about 11,000 years ago (ya). But what you also need to consider are the two ice sheets that blocked passage further south until around 12,000 ya. The land bridge is only half of the story!

The Bering Land Bridge was available throughout the last ice age, from around 126,000 yrs ago (126 kya), but temperature and weather conditions fluctuate during that (enormous) time period. In colder periods, the land bridge was more exposed- because the world’s water was in the glaciers at the caps, and not in the oceans (think: low glaciers = high water; vs. large glaciers = low water.

After around 21,500 yrs ago, glaciers begin their slow retreat, as the most recent coldest conditions of the ice age come to a close. Conditions for a Beringian crossing are optimal from 22 to around 19,000 years ago, and the bridge is certainly still passable at 15,000 years ago. Technically, it’s still possible to cross as recently as… around 11,000 yrs ago, just not as exposed (and optimal) as previous. Sometimes folks put this number at 12,000 ya, or even a bit after 11,000 ya, so different researchers will state slightly different dates here, but all give the latest exposure of Beringia somewhere around 13-10,500 yrs ago.

More than likely, they’re using a molecular clock model to estimate when wolves first diverged from earlier species (comparing the DNA of wolves and other species, and counting back using the estimated rate of mutations to pinpoint the last common ancestor). I’m not familiar with the paleozoological fossil record- only the human one- so I don’t know what fossil evidence we have of wolf precursor species at 750kya. But for many species, the genetic method is pretty sound, depending on if the rate of mutations for that corner of kingdom animalia, and for specific areas of the genome, are well estimated.

Continents looked not all that different 750,000 yrs ago, but ancient wolf ancestors certainly were equipped to migrate across earlier than humans. This interactive map from Princeton shows human migration from 100kya: http://assets.press.princeton.edu/chapters/haywood/s2_9519.pdf. This article discusses dire wolf fossils in general, though dire wolves are not more closely related to modern wolves than other species, such as the jackal: https://www.nps.gov/articles/000/dire-wolf.htm. This page presents further discussion and references of species related to modern wolves: https://www.floridamuseum.ufl.edu/florida-vertebrate-fossils/species/canis-dirus/

None of the rest of this has to do with wolves- only humans. This is where things (to me) get interesting.

Getting across into North America is only half of the story. From 24,000 ya, the Laurentide and Cordilleran ice sheets blocked passage overland heading south from the arctic into the subarctic regions, and into what is now southern Canada and the U.S. Basically, you had two large ice sheets that begin to recede as conditions warm- and up until 13,000 yrs ago, the route between them (much of which is now called the MacKenzie Borderlands) is not passable to humans (not sure about other animals, but humans are following the animals… until 13kya, the route south was blocked by ice.

The earliest archaeological evidence of humans in the MacKenzie borderlands isn’t until around 12,000 ya. As a result, early models of human migration into North America presumed that the earliest that humans could have migrated south from the arctic is around 12,000 ya at the earliest (the ice free corridor hypothesis). Up until relatively recently, it was thought that boat travel south along the Pacific Northwest was impossible because… boats were not an accessible technology? (I suppose the argument was that there wasn’t any direct evidence of boats, so… no boats? Ignoring that Indigenous peoples have long used kayaks in the region; humans were certainly smart and resourceful enough to make them; and that lack of preservation may be hampering interpretation- a big consideration in archaeology- but still, No Boats was the ruling, impenetrable interpretation until… maybe the 1980s-1990s?). Rather short sighted and, erm, dismissive (and racist), if you ask me.

It was also a bit of a pickle, as stone tools known as Clovis points and Folsom points (quite large and difficult to produce bifacial stone spearpoints, possibly attached to wooden handles) are found in contexts dating to between 13,200 and 11,900 yrs ago, over much of North America.

That interpretation (and pickle) held sway until evidence of butchered megafauna (large animals like mammoth and mastodon) were recovered from contexts dated to 14,200 ya and 14,800 ya in what is now Wisconsin (Schaefer and Hebior sites, respectively). There is also a mastodon with a spearpoint still embedded in the bone, dated to 13,800 ya (the Manis mastodon) from Washington State. And finally: evidence from Paisley 5 Mile Cave, from which were recovered… significant (more than 65!) coprolites, or fossilized poop. Identified by DNA as human. So humans were here, pooping in caves, and hunting, in contexts that pre-date migration in-between the ice sheets. And with this, hesitation about boats dissolved.

At the same time, the past few decades have seen numerous contexts in Mesoamerica and in South America dated to 12,000-13,000 ya, or even 14,000 ya. The most impressive of them all is Monte Verde in southern Chile, dated to around 15,000 ya (and they have recently released evidence claiming dates earlier still, perhaps up to… 20,000 ya, though I haven’t had a chance to scrutinize it yet- it’s not just lab-reported carbon 14 dates that you rely on. You also need a careful scrutiny of what material was dated, and how, and if it was contaminated, or if layers were mixed up, etc). So we absolutely know that humans migrated over earlier than the ice sheets allowed- and for part of that journey, it is highly likely that they used boats.

Now, we’re also seeing mounting evidence that humans traversed the Pacific Ocean in boats from around 15,000 ya. I’m less familiar with the particulars of that evidence, but reports about it have been circulating in the news for the past two years. Keep an eye on this question, because it’s constantly changing, and is quite an exciting area of science.

We’re also reconsidering the idea of a single crossing. The Beringia Standstill hypothesis proposes that humans were living on the Bering Land Bridge, migrating back and forth, for perhaps a few millennia, if not longer. For more info see: Stone 2019.

Dates from: Kenneth Feder 2016. The Past in Perspective. Oxford. (Textbook with citations I’ve scrutinized for use within my classes);

For more info about the ever-evolving reconstruction of human migration in Siberia and Beringia through genetic information, see: Anne C. Stone 2019, ‘Human Lineages in the Far North’, Nature 570 (Stone also uses 126kya for the earliest date of the last ice age; this date fluctuates in other sources, including to around 110kya).

Paisley 5 Mile Cave coproplites, see: M Thomas P Gilbert et Al. 2008. DNA from pre-Clovis human coprolites in Oregon, North America. Science 320(5877): 786-9.