North America

The initial habitation of the Americas has gotten a lot of press attention in the last couple of weeks.

National Geographic gave us a report on skeletal remains from an underwater cave in Yucatan, called Hoyo Negro ("Skull in underwater cave may be earliest trace of First Americans"). There's no date yet for the human remains, which are associated with megafauna -- but no reason at all to go with the news story's "15,000-20,000 years ago," that's just sensationalism.

Last week, Science published a report on a child cremation burial from Alaska dating to 11,500 years ago [1]. In the paper, Ben Potter and colleagues compare the Alaskan site (Upward Sun River Site -- USRS) to a site in the western part of the region that was once Beringia, but now is on the Siberian side of the Bering Strait:

Only one other ancient burial site is known for Beringia: Ushki Lake 1, in Kamchatka, Russia (34–37) (Fig. 1). Ushki Lake 1, Level 7 (Ushki L7) (~13,000 cal yr B.P.) contained an adult burial associated with bone beads in a rock-lined ochre-filled pit separated from the house structures. Ushki Lake 1, Level 6 (Ushki L6) (~12,000 cal yr B.P.) is roughly contemporaneous with USRS Component 3 and contains two unburned burials of children within two separate houses (35, 36). One child burial contained ochre, a pendant, a mat of lemming incisors, and numerous microblades and wedge-shaped cores (the second burial is undescribed) (35). Thus, the USRS burial context is more like Ushki L6 than L7. This replicates technological linkages between continents: Diuktai Culture of Ushki L6 is comparable with the Denali Complex, which dominates the record from 12,000 to 6000 cal yr B.P. in interior eastern Beringia (24, 38), whereas the Ushki Culture of Ushki L7, associated with stemmed points and lacking microblades, arguably has no direct counterpart in North America [(39), but see (34)].

That reference to the stemmed points becomes important in the next paper, published in Science this week by Erlandson and colleagues [2]. The report is a description of a mixed archaeological assemblage from the Channel Islands of California, with a few artifacts from a shell midden dating to 12,200 BP [2]. The date is not all that early, not earlier than Clovis. It's interesting because it seems to further the evidence for a distinct archaeological tradition in the West, with inland occurrence and possible connections to South America.

If Arlington Springs [skeletal remains dating to 13,000 BP] is included, the earliest Paleocoastal Channel Island sites are contemporary with Clovis and Folsom sites of the continental interior (6, 8, 20). The island sites provide evidence for Terminal Pleistocene seafaring, island colonization, and a diversified maritime economy, adding to the variability of Paleoindian adaptations in the Americas. The stemmed points and crescents dated as early as 12,200 cal BP link these early island assemblages to those found in interior Western Pluvial Lakes Tradition (WPLT) sites found around many lakes and marshes in North America’s Far West (15). Stemmed point fragments have also been recovered in the basal levels of Paisley Caves, dated to ~14,300 cal BP (21), and the Paleocoastal stemmed points and crescents from the Channel Islands seem unlikely to be descended from Clovis. Such WPLT assemblages may provide a logical technological link among Terminal Pleistocene stemmed point traditions of Northeast Asia (22), the Pacific Northwest, and possibly early stemmed point traditions widely distributed in South America (23).

The Clovis industry was a very short-term phenomenon, and spread across an area of North America that makes an unlikely link to the rest of the Americas. Seems more like a cul-de-sac in some sense. Movement down the western coast makes more sense, but the cultural traces of early Paleoindians have been scarce. But these seem to be adding up to something -- the stemmed point in Paisley, now the earliest site with biological evidence of humans in the Americas, is interesting in this regard. It's not a radical revision of the timeline; this is all about a relatively short period of pre-Clovis occupation, maybe 2000 years as we understand it now. The research is beginning to make more connections among early occurrences, making them seem more like a system than like outliers.

UPDATE (2011-03-07): A reader (who should know) chides me for describing Clovis as a "cul-de-sac" industry, noting the distribution of fluted points is much more widespread. Another reader expresses some interest in the ecological setting of these stemmed points across the broader West. I will return to the issue soon, which deserves a fuller review than this.

References

In 2007, R. B. Firestone and colleagues published evidence of an extraterrestrial impact, roughly coincident with the onset of the cold climate event known as the Younger Dryas. This event, around 12,900 years ago, is around about the time of some (but not all) megafaunal extinctions in North America, it is also around the time (but not precisely) of the Clovis culture. The paper argued that the impact event may have "contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America".

Last year, I reported on widespread dissatisfaction with this impact hypothesis. Some critics didn't think that there was any evidence of megafaunal trauma from the impact, some didn't think that the dates matched any "adaptive shifts", and in particular the end of the Clovis culture.

And then others didn't think that there had been an impact at all. These were in some ways the most worrisome, because they directly questioned the supposed evidence in support of an extraterrestrial event -- "microspherules" of magnetic material, clustered in sedimentary contexts at precisely 12,900 years ago in sites across much of North America.

Now, in the Proceedings of the National Academy of Sciences (where Firestone and colleagues originally published their observations), Todd Surovell and colleagues have published a remarkable paper that tests the Firestone impact hypothesis: "An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis." Most critiques attempt to find an alternative explanation for a set of original observations. In this paper, Surovell and colleagues merely attempt to replicate the original observations at multiple sites, and fail -- as their abstract tersely states,

We were unable to reproduce any results of the Firestone et al. study and find no support for Younger Dryas extraterrestrial impact.

Just like that -- it's about as hard-hitting as you're going to see in a scientific research paper.

Of course, this paper only examined one out of a number of observations that Firestone and colleagues had adduced in support of the impact hypothesis. But in the introduction to their paper, Surovell and colleagues reference several other recent studies that re-examined other aspects of the evidence:

A series of critiques of the original Firestone et al. article (1) have been published recently (8-10). Pinter and Ishman (8) argue that the suite of markers used to indicate impact are inconsistent with "any single impactor or any known event." Furthermore, they provide alternative explanations for many of the observed marker peaks. For example, glassy and metallic microspherules are known components of atmospheric dust derived from the constant influx of micrometeorites. An independent evaluation of the charcoal evidence was recently published by Marlon et al. (9). Examining concentrations of charcoal from 35 pollen cores across North America, they found no evidence for large-scale, continent-wide wildfires specifically associated with the onset of the [Younger Dryas].

In the current case, the results are very simple: they went looking for a spike in the number of impact-generated particles coincident with the Younger Dryas. They looked at seven sites with long and continuous records of sedimentation across that interval. They found the supposed impact-generated particles, but not patterned with any kind of spike.

They suggest a different model for the presence and accumulation of the magnetic particles:

Alternatively, it may be that the presence, absence, and relative abundance of magnetic materials, especially the spherules, is due to characteristics of the parent material and depositional environment instead of some sort of continent-wide extraterrestrial process. The characteristics of the local depositional setting before, during, and after 12.9 ka have not been addressed by the proponents of the impact hypothesis. The zones producing the YDB ‘‘impact markers’’ are typically associated with soils (stable surfaces) or shifts in the depositional environment (e.g., alluvial to lacustrine conditions at Blackwater Draw, Lubbock Lake, Murray Springs, and Lake Hind; buried soils in the Carolina Bays and at Lommel, Belgium).

One might imagine atmospheric particles accumulating on stable paleosols over long stretches of time, generating a local spike in the number of such particles in the stratigraphic column. In any event, the data presented here don't bear out the hypothesis of any unusually large impact event.

I'm not a geologist, and I have no special insight into the analyses here, beyond reading the charts. But remember that the impact hypothesis made a tremendous media splash. Maybe more damaging to the scientific side of things, the hypothesis that the Younger Dryas cold period came from an extraterrestrial force, made it seem for a moment less necessary to investigate terrestrial sources of cooling at the terminal Pleistocene. The science will correct itself, but the public perception of the climate changes at the end of the Ice Ages will need quite a bit more nursing to get a more realistic perspective on the story.

References:

Firestone RB and lots of others. 2007. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proc Nat Acad Sci USA, 104:16016-16021. doi:10.1073/pnas.0706977104

Kerr RA. 2008. Experts find no evidence for a mammoth-killer impact. Science 319:1331-1332. doi:10.1126/science.319.5868.1331

Surovell TA, Holliday VT, Gingerich JAM, Ketron C, Haynes CV, Jr, Hilman I, Wagner DP, Johnson E, Claeys P. 2009. An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis. Proc Nat Acad Sci USA (early) doi:10.1073/pnas.0907857106