How do we know that Neanderthals were nearly all right-handed?

A while back I wrote about my visit to the Croatia Natural History Museum, where curator Dr. Davorka Radovčić kindly gave three of us a several-hour look at Neanderthal bones from the nearby location of Krapina, one of the most fruitful Neanderthal sites known. At the time I mentioned there was evidence that most Neanderthals were right-handed, but I didn’t really explain why. Now Davorka has sent me two papers (references and links below) that show how we know this. I’m going to write mostly about the Lozano et al. paper (free with the legal UnPaywall app), which tells the tale up to the present. If you can’t get either or both of these papers, email me and I’ll send them.

It is in fact true that about 90% of Neanderthals were right-handed, and that’s the same as present-day H. sapiens sapiens, even though Neanderthals aren’t really the ancestors of modern humans (we do, however, carry some of their genes). That probably means that the common ancestors of our two subspecies—I consider Neanderthals as H. sapiens neanderthalensis, a subspecies of H. sapiens—were also right handed. And indeed, chimpanzees (though not bonobos) are 49% right-handed and 29% left-handed, with 22% of individuals “ambiguous”.

But new data also shows that our ancient ancestors—before the split between modern H. sapiens and Neanderthals, were also right-handed. How did they do this?

It doesn’t come from looking at arm robustness in fossils, for that doesn’t work, nor does it come from looking at brains (as seen in crania), as that doesn’t work, either. It comes from looking at incision marks on the teeth made when a hominin is holding something in its mouth and cutting it—cutting it with the dominant hand. It looks like this (figures from the Lozano et al. paper:

 

Figure 1 [All captions from figures] Demonstration of how marks were likely made on the incisors and canines. A right‐hander pulls down with a stone tool, cutting through the object held between the anterior teeth. Occasionally, when the tool accidentally strikes the tooth’s surface, it leaves a permanent striation on the labial tooth face. Repetitive marking of the labial face allows for the assessment of which hand was used in this bimanual task

Sometimes you’ll hit your teeth with the cutting tool, and the striations (scratches) that this leaves on your teeth—in particular the incisors and canines, but especially the upper incisors—tell you what hand is doing the cutting. Try it!  Imagine you’re holding a piece of meat, or a skin, in your teeth and cutting it with your right hand (if you’re right handed, that’s what you’ll be doing). If you hit your teeth with the cutter (a sharpened stone), it will make a scratch from lower right to upper left, because the tool  will be oriented that way (hold a piece of paper in your mouth and pretend you’re cutting it). If you’re using your left hand, the cuts will be from lower left to upper right. And since you know where in the jaw the teeth are, you can determine handedness if there’s a consistent direction to the scratch marks.

Sometimes the marks will be horizontal or vertical, and sometimes they’ll be made not by humans but by taphonomic (preservation) forces, like sand scratches. You can deal with the latter by using marks only on the front edge, comparing them to those on the rear of the tooth, which should be subject to the same taphonomic modification. Also, you want not he percentage of teeth that show handedness, you want the percentage of individuals that show handedness. To deal with the first and last problem, the authors used these methods:

Thus, striations were separated into four orientation categories: horizontal (H: 0°–22.5°, 157.5°–180°), vertical (V: 67.5°–112.5°), right oblique (RO: >22.5°–<67.5°), and left oblique (LO: >112.5°–<157.5°). This underestimates the number of right or left handers; for example, an oblique mark of 21° would be classified as horizontal, so if the intervals were expanded the tooth being examined would have come from a right‐hander. However, since most studies have not published the raw data and have used the Bermúdez de Castro et al. intervals, we also used them.

Many of the teeth are isolated, especially in the Krapina sample. For this site we used Wolpoff’s reassembled tooth sets, each of which he labeled as a Krapina Dental Person (KDP). His tooth associations were based on similar morphology, occlusal wear, and interlocking interproximal facets, not on the presence of labial scratches. It is unlikely that any of the KDPs in our sample can be grouped together into a smaller number of individuals.

They also tested the “direction” hypothesis by making mouth guards that could be scratched, but also by looking at mouth guards with embedded teeth, as well looking at present day hunter-gatherers and Inuits. These showed directional striations consistent with observed handedness.

Finally, the authors analyzed several samples of hominin teeth: the total sample included five different types of humans (Homo habilis [OH 65, 1.8 million years old], Homo antecessor [from Gran Dolina, 860‐936 kya] the Sima de los Huesos fossils [430,000 years old probably ancestors of Neanderthals], European Neandertals, and modern Homo sapiens).

Here’s the earliest one, the OH-65 Homo habilis, 1.8 million years old. The graph below gives the directions of the scratches, and the predominance of the red bar (right oblique) over the blue bar (left oblique) shows that this individual was probably right handed:

OH‐65 shows a concentration of striations on the labial faces of the anterior teeth. These are visible to the naked eye. Microscopically, they conform to the striations found in much later hominids. The striations are mainly confined to the left and right I1s, the right I2, and right C1. Right oblique scratches predominate, leading to the identification of OH‐65 as a right‐hander. (n = number of striations per category) [Color figure can be viewed at wileyonlinelibrary.com]

The Gran Dolina H. antecessor individual didn’t have enough scratches to be identified but here’s the tooth of a right-hander from about 400,000 years ago (the Sima de los Huesos site):

Here are three Neanderthal teeth with the striations emphasized: the first is a left-hander and the other two right-handers based on the numerical predominance of directionally oblique scratches:

Here’s the final table that tabulates handedness. The earliest hominin was right handed, as were all 15 of the Sima de los Huesos individuals, suggested that by at least half a million years ago, right-handedness predominanted in hominins. The Neanderthals are the ones from Krapina down, and they show a 90% frequency of right-handedness, similar to humans today.

I should add that they also found directional scratches over old directional scratches (the enamel partly heals itself), so the directionality continued throughout the life of an individual, and they find directionality in teeth estimated to be from 10-year-old children as well. Since they didn’t have knives, I suspect much of this involved cutting meat, but also animal skins.

It looks as if since the hominin lineage branched from the lineage leading to chimps and bonobos, we’ve been largely right-handed: about 90%. It would be nice to have earlier fossil data, but this is pretty damn good.  I think the methodology, with its controls and observations of modern humans, is sound. The authors conclude:

We contend that the handedness data reviewed here shows that right‐handedness extends deep into the past of our species. The modern right‐handedness frequencies in earlier European human fossils from Sima de les Huesos and new specimens from the Early Pleistocene of China and Africa suggest that handedness stretches back well before the appearance of Homo sapiens. European Neandertals represent the biggest samples and continue this pattern, showing a right‐to‐left hand ratio identical to that among living Homo sapiens. In our view, the unique 9:1 ratio of right to left handers appears well before the emergence of modern Homo sapiens and is typical of our genus wherever and whenever it is found.

One question remains:

Why does there have to be a dominant hand? Why can’t humans (or those animals that show handedness) be equally dextrous with both hands?

This may be a byproduct of our brain structure (the authors posit that it’s a result of brain lateralization for language or other reasons), or there may be some other reason we don’t understand why one hand must predominate (and it can’t be random because most of us are righties, and there’s a genetic component to that). Who knows? But we do know that most of our ancestors were right-handed—at least according to these data and the data from the Fiore et al. paper.

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Lozano, M. et al. 2017. Right-handed fossil humans. Evol. Anthropol. 26: 313-324.

Fiore, I., L. Bondoli, J. Radovčić, and D. W. Frayer. 2015. Handedness in the Krapina Neandertals: A Re-Evaluation. PaleoAnthropology 2015:19-36.