This is Why … campfire smoke may have played a surprising role in our evolution

(campfire image from Shutterstock)

Like me, you may have spent a significant amount of time around a campfire this past long weekend. Singing songs, roasting marshmallows, debating the optimal wood configuration, discussing the connection between campfire smoke and eating placenta …. No? Not the last one? Well buckle your seatbelts, kids, let’s dive right in!

Although there are many who love the lingering smell of campfire in their hair and clothing, there is plenty of science to say that prolonged exposure to campfire smoke is not good for us. Smoke from wood-burning fires contains significant quantities of known health-damaging substances, including several carcinogenic compounds. So, avoiding wood smoke is definitively a worthy goal around the campfire. But how? A study published in Nature Scientific Reports in 2015 by Adrian Bejan from Duke University concluded that a 1:1 base to height relationship is ideal. He asserts that humans of all eras have been relying on this design “unwittingly”, instinctively building fires in such a way as to generate the most heat (and least smoke) once our hominid ancestors figured out how to harness this energy source.

The use and control of fire by our ancestors is a key event in our evolution, with theories that this development influenced our social structures, our geographic expansion into cooler climates, even the way our brains evolved. While exploring the question of “why does campfire smoke follow me around?” for a friend[1], I stumbled upon an even more intriguing evolutionary campfire-smoke theory: the nastiness of campfire smoke may be the reason that humans are essentially the only terrestrial mammals that do not eat placenta and amniotic fluid after childbirth.

An sketch of what our ancestors’ cave might have looked like, from Shutterstock

It turns out that humans are quite unusual in our rejection of the practice of ‘placentophagia’. This behaviour is regularly observed in ALL nonhuman primate species, as well as in the vast majority of more than 4000 terrestrial mammals currently in existence. So, why not in humans? (other than “eww, gross”) There are plenty of animal studies to show the benefits to the mother and the offspring for ingesting placenta and amniotic fluid (via cleaning the baby) – so why would humans not do it? Not only do we not do it now … an extensive anthropological literature search of up to 300 societies around the world turned up no evidence of the behaviour in any pre-industrial human society. Of course ‘absence of proof is not proof of absence’, but it is intriguing! Since present-day nonhuman primates exhibit the behaviour, there must have been a point in our evolution in which we stopped doing it. In 2012, Sharon Young, Daniel Benyshek, and Pierre Lienard from the University of Nevada published their theory that it may have stopped due to the unhealthy effects of a universal practice unique to humans – the habitual use of controlled fire.

Mumma deer, cleaning her newborn fawn, from Shutterstock

The placenta is a filter to protect the baby. Hominid mums spent a lot of time around the fire – inhaled lots of smoke – filtered the bad stuff out. Bad stuff accumulates in the placenta and it becomes a health risk to eat. The costs now outweigh the benefits that other species get from eating the placenta and amniotic fluid that, in their case, is not accumulating bad stuff since they aren’t hanging around the campfire. In other words, natural selection may have eliminated the practice of placentophagia among humans or our direct human ancestors as we spent a lot more time inhaling wood smoke.

So, the next time someone around the campfire complains about getting smoke in their eyes (and lungs), you can astound them with this fascinating connection to placentophagia. Depending on your company, you might just end up with a greater share of the marshmallows as a result! You’re welcome!


Why humans build fires shaped the same way; Adrian Bejan, Scientific Reports volume 5, Article number: 11270 (2015)

Woodsmoke Health Effects: A Review; Luke P. Naeher, Michael Brauer, Michael Lipsett, Judith T. Zelikoff, Christopher D. Simpson, Jane Q. Koenig & Kirk R. Smith, Inhalation Toxicology: International Forum for Respiratory Research, Volume 19, Issue 1, 2007, pages 67-106

Placentophagia in Humans and Nonhuman Mammals: Causes and Consequences; Mark B. Kristal, Jean M. DiPirro, Alexis C. Thompson, Ecology of Food and Nutrition (2012) 51: 177-197

The Conspicuous Absence of Placenta Consumption in Human Postpartum Females – The Fire Hypothesis; S. M. Young, D. C. Benyshek, P. Lienard, Ecology of Food and Nutrition (2012) 51: 198-217

Fire in the Plio-Pleistocene: the functions of hominin fire use and the mechanistic, developmental and evolutionary consequences; Laura Attwell, Kris Kovarovic, Jeremy Kendal, Journal of Anthropological Sciences (2015) 93: 1-20

[1] It doesn’t, that’s not how air convection works. Let me know if you’re interested and I can write a separate piece on this subject!

Published by joanneomeara

Professor, Department of Physics, University of Guelph

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: