Pleistocene Walnuts (Juglans sp.)

Walnuts first appear in the fossil record during the Eocene, 45 million years ago. Some scientists believe this is when walnut trees first began to diverge into different species, however, this belief is inconsistent with the results of a recent genetic study. Today, there are 21 species of walnuts, but the genetic evidence suggests they didn’t begin to diverge from a common ancestral species until about 1.5 million years ago. Some scientists believe 1 walnut species occurred from North America across the Bering Land Bridge through Eurasia before Ice Ages began. The genetic study (referenced below) determined species divergence began after Ice Age climate conditions isolated different populations of walnut trees. The authors of this study suggest the 15 known fossil species of walnuts, dating to before the Pleistocene, are actually just 1 species that lived in a multi-continent zone of equable forest. Another odd finding of this study determined there were no population declines in walnut populations during the Last Glacial Maximum when temperate forest zones were replaced in northern latitudes with boreal forest zones dominated by spruce. Also, plant diseases appear to be a greater or equal influence on walnut populations than climate.

The 21 species of walnuts all belong to the Juglans genus within the Juglandaceae family which also includes the hickories. The English walnut (Juglans regia) is the commercially grown walnut found in grocery stores. This species originated in Iran. Ice Ages caused the extirpation of walnut trees in most of Europe. Two species of walnuts are native to eastern North America–the black walnut (J. nigra) and the butternut (J. cinerea). The latter species is in serious decline due to a disease pathogen. Black walnut wood is prized by furniture makers, and the nuts are a little tastier than English walnuts. The shells are much harder to crack. I find it necessary to use a meat mallet on the hard floor…standard nutcrackers won’t work. Black walnuts are not grown commercially on a large scale, and most of the ones found in grocery stores are gathered from the wild. There are also different species of walnuts found in California, Texas, the West Indies, South America, China, and Japan.

I could have gathered bushels of black walnuts at Chickamauga Battlefield Park. I took as many as I could fit in my pockets.
There were many black walnut trees on the edge of the woods here.
This species is prized for its wood used in furniture making, but the nuts are tasty too.
The shells can be used as a brown coloring agent. Removing the husk can stain everything they touch, so use rubber gloves.
Black walnuts are hard to crack. I use a meat mallet and the floor. Before they can be used, they must be seasoned. I left mine in a warm car for 2 weeks.

Walnuts are an excellent source of nutrition. They are 14% carbohydrate, 15% protein, and 63% fat. The nuts are high in B vitamins, minerals, and the healthy kind of fat also found in salmon and other fatty fish.


Bai, Wai-Ning; et. al.

“Demographically Idiosyncratic Responses to Climate change and Rapid Pleistocene Diversification of the Walnut Genu Juglans (Juglandaceae) Revealed by Whole Genome Sequences”

New Phytologist November 2017

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