Younger Dryas Impact Hypothesis: Part 3

Phew, finally at Part 3! These are the last two areas of debate surrounding the YDIH I'm going to cover, but there are a few more that go deeper into the geology. The Holliday et al (2014) has quite a bit more on the geology as does the Pinter et al., (2011) paper. 

Megafauna extinctions

Does the YDIH explain the megafauna losses better than human or climate models? As mentioned in Part 1 of this post, the YDIH is claimed to overcome many of the more uncertain aspects of the climate or overkill hypotheses (Firestone et al., 2007). The YDIH, however, also runs into some issues when it is applied to the megafaunal extinctions.

The problem is that a giant impact in North America should, intuitively, cause the highest number of extinctions in North America. Even on the American continent this is not true, with around 50 mammal genera lost in South America and 33 in North America  (Barnosky et al., 2004). 

This problem gets bigger when you consider the extinctions that occurred world wide. Europe, Africa and Australia suffered megafaunal losses (see my Causes of Late Pleistocence Continental Extinctions post!). You could imagine that the impact was big enough to cause these widespread extinctions, but then how did any megafauna survive in North America? The cougar, grey wolf, bison, musk-oxen, elk and tapirs are all end-Pleistocene survivors, to name a few (Holliday et al., 2014). 

These aren't ecologically confined examples either. The survivors from North and South America present a wide range of life histories and ecological niches (Holliday et al., 2014). So it can't be that one environment was less affected, allowing a certain group to survive. 

Moreover, the fact that megafauna survived on Wrangel island, Russia and St. Paul island, Alaska while megafauna on the neighbouring continents did not, doesn't make sense in the context of an impact event (Holliday et al., 2014). 

Clovis decline and cultural shift:

If the megafauna were affected by an impact, then the humans should have been too (note: humans are considered part of the mammalian megafauna group (Barnosky, 2008)). Humans in North America at that time belonged to the Clovis culture (Haynes, 2008). The YDIH suggests that this group underwent an adaptive shift combined with a population decline (Firestone et al., 2007). This is claimed to explain the shift in culture at 12,900 BP, as well as an archaeological gap immediately following the Clovis period in which no human artefacts are found (Firestone et al., 2007).

YDIH proponents point out that sites containing both Clovis and post-Clovis are rare (Firestone et al., 2007), implying a disruption in settlement or landscape use, as the result of an impact. Interpreting this as a population collapse, however, is problematic because most Palaeoindian sites were not re-used (Holliday et al., 2014), and so the majority of these sites, including Clovis, also lack immediately succeeding occupations/ land use. Where multiple occupations do occur at such sites, stratigraphic gaps between them are readily explained by geomorphic processes (Holliday et al., 2014). 

Also, there is an issue with defining exactly what is and what isn't Clovis. Clovis culture is generally defined by the characteristic shape of the arrow heads its people made (Howard, 1990), but even within this (somewhat arbitrary) group there is considerable variation, e.g. between different populations (Smallwood, 2010). 

From Simthsonianmag
Variation in Clovis points

Where Clovis and post-Clovis sites are well defined chronologically (that is, people assign definite time boundaries to the two groups), the archeological and stratigraphic records fail to provide evidence of a population collapse (Holliday et al., 2014). On top of this, calibrated radiocarbon ages show continuous occupation across the time of the impact event rather (Holliday et al., 2014). 

Finally, the apparent end of the Clovis culture is probably actually an evolution of parts of a tool assemblage (a common occurrence in the global archaeological record). It might therefore be hard to argue that a change is tool types is due to an environmental disaster (Holliday et al., 2014). 

So the YDIH may be an unnecessary solution for archaeological problems that don't exist  (Holliday et al., 2014). (And it doesn't really help with the environmental or megafauna problems either)... In summary, a large proportion of the lines of evidence suggested have been non-reproducible (a very bad thing for a scientific theory) (Pinter et al., 2011). The evidence left over seem to be a result of non-catastrophic mechanisms and terrestrial processes (Pinter et al., 2011). There's a fair bit of bad science surrounding the YDIH, so I, personally, am unconvinced. But read around, and decide for yourself.