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It’s rude to ask someone their weight, yet we have to make an exception for Lucy. This ~3.2 million years old fossil represents a key moment in human evolution. So researchers have been studying everything about her since she was found in 1974. Including how much Lucy weighed.

And it turns out she was surprisingly dainty, weighing in less than a labrador.

Labrador out here looking majestic AF
CC BY-SA 3.0,
https://commons.wikimedia.org/w/index.php?curid=511957

How do you weigh a fossil?

These adorable findings come from research by Brassey et al., published in the Journal of Human evolution. Studying the weight of a fossil is hard since most of the individual has rotted away. So, these researchers first created a 3D model of the famous fossil. Then, they effectively shrink-wrapped their reconstruction using a process known as convex hulling1.

This, in of itself, doesn’t tell us much. However, there is a correlation between the volume “shrink-wrapped” and the weight of the individual (at least in modern primates)1.

A baboon skeleton (left) convex hulled (right)1.

To be fair, researchers have been investigating the weight of Lucy and other ancient members of the human family (called hominins) for years. However, most of these methods rely on making extrapolations from the size of the bones; which comes with some margin of error. Particularly given these extrapolations are based on data from different species1.

However, when Brassey et al. used their new 3D method to calculate Lucy’s weight, she clocked in at a measly 20.4 kg (45 lbs). That’s less than the 29 kg of your average labrador, putting her on par instead with a British bulldog (although the upper range of these estimates would make her labrador sized).

If these figures seem shocking to you, you aren’t the only one. Brassey et al. were also surprised by this estimate of Lucy’s weight, which is much lower than the previous calculations1.

Lucy after convex hulling1. Which it turns out makes her look like a PS1-era video game character

Doubting how much Lucy weighed

This estimate isn’t just small compared to dogs. They would also make Lucy one of the lightest primates known, given her stature.

As such, Brassey et al. are cautious about their findings. They note that their calculations of weight from the shrink-wrapped volume are based on modern primates, so that might introduce an element of uncertainty. Further, since we don’t have the complete Lucy fossil there’s the possibility their 3D reconstruction is inaccurate.

No reason for this, just thought you could all do with another dog pic.
By RichardF at English Wikipedia. – Transferred from en.wikipedia to Commons., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=2122124

Nevertheless, there’s also the possibility that Lucy was just on the little side.

Notably, there’s a growing body of evidence that her species, Australopithecus afarensis, featured fairly extreme size differences between the sexes. Could Lucy’s weight be another symptom of this sexual dimorphism?

More research will hopefully shed light on this. And, perhaps more importantly, give me more opportunities to post dog pics.

References

  1. Brassey, C.A., O’Mahoney, T.G., Chamberlain, A.T. and Sellers, W.I., 2018. A volumetric technique for fossil body mass estimation applied to Australopithecus afarensis. Journal of human evolution115, pp.47-64.

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