Megafauna Extinction Linked To Human Ancestors

One of the glaring facts of human prehistory is the correlation of the rise of humans and the decline of megafauna—big game.  This pattern continues today, with simultaneously accelerating extinctions of large animals and booming human populations.

Many of us think this pattern is no coincidence.  We know that people will hunt anything and everything to extinction.  All those nasty little apes with their pointy sticks probably did in the giant sloths and other big beats.

Cautious souls reserve judgement, because it is certainly possible that some third factor, such as changing climate, led to both more humans and fewer game animals.  Just because we are killing everything in sight today doesn’t mean that humans wiped out ancient all the ancient fauna.

Given the sparse and uncertain data about exactly when and where humans lived, and when and where species went extinct, there is only circumstantial evidence one way or the other. Correlation is not causation.

So, what role did we humans and our cousins play in the large die off at the end of the Pleistocene?


This spring, a group of researchers published a study of mammalian extinctions and human expansion from the last 125,000 years [2].  The study worked with a dataset that includes mammalian body size distributions and biodiversity over time.

“We investigated the influence of these emerging and increasingly sophisticated hominin predators on continental and global mammalian biodiversity over the late Quaternary” ([2], p.310)

Of particular interest are five broad periods of time corresponding to the expansion of hominins (humans and cousins).

The analysis showed a clear relationship between size of the animals and likelihood of extinction, especially in the earlier periods.  This means that larger animals were consistently wiped out.  Notably, a similar analysis for periods before the Pleistocene (before homonins) do not show this pattern.  (The pattern is less visible in recent times, likely because everything is being wiped out at the same time.)

“As Neandertals, Denisovans, and humans spread across the globe over the late Quaternary, a highly size-biased extinc- tion followed, a pattern distinct in the Cenozoic mammal record. The subsequent downgrading of body size was severe and differentially targeted herbivores.”

This pattern is consistent with human hunting behavior, and is seen at the precise periods when humans were expanding.

One interesting conclusion from this data is that this pattern began very early, and, indeed, before Homo sapiens evolved from earlier Hominids.  This implies that our ancestors have been big game hunters from the beginning, and have been significantly impacting big game from forever.  (Nasty little apes with pointy sticks…)

This pattern has abated in recent ages because humans have come to dominate the Earth, and domesticated animals have replaced wild animals.  The study projects into the future, assuming that threatened species die out. In the future projections, the extinctions extend into smaller animals, indeed, nearly all wild mammals.


I was inspired to make my own plots from some of their data (I drew from Table S1, Supplemental materials).  These diagrams make plain the extremely rapid decline in animals with large body mass, and the key temporal pattern:  extinctions began very early in Africa and Eurasia, spread out to Australia and then the Americas.  This is, of course, the path of human occupation.

I drew arrows suggesting when humans arrived and expanded.  Note that these marks are impressionistic, dates and scales of human occupation are not well established.

(In these diagrams the last point is the projected future extinctions in then next 200 years, which is a precipitous drop.)

From [2] Supplemental Materials, Table S1. Horizontal axis is years (1000s), vertical axis is median body mass for surviving species. Lower median body mass means fewer large animals. Arrows suggest when major human infestations may have first occurred.

  1. Christopher Joyce, New Study Says Ancient Humans Hunted Big Mammals To Extinction, in All Things Considered. 2018, National Public Radio: Washington, DC. https://www.npr.org/sections/thetwo-way/2018/04/19/604031141/new-study-says-ancient-humans-hunted-big-mammals-to-extinction
  2. Felisa A. Smith, Rosemary E. Elliott Smith, S. Kathleen Lyons, and Jonathan L. Payne, Body size downgrading of mammals over the late Quaternary. Science, 360 (6386):310-313, 2018. http://science.sciencemag.org/content/360/6386/310.abstract

 

 

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