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One of the most important factors in the evolution of humans and other hominins is their relationship with food, and how it has changed over the millennia.

There are some foods that we can barely imagine living without, but that are quite recent additions to our diet. Take wheat, which we use to make bread, pasta, cake and more. Farmers first grew it perhaps 10,000 years ago, in the Middle East. There is archaeological evidence of somewhat earlier: up to in the Levant region. Given that our species, Homo sapiens, is around 300,000 years old, and hominins have existed for 7 million years, that isn鈥檛 long ago.

In contrast, hominins have been eating big game animals for much longer. While the earliest hominins seem to have mostly eaten plants, there is evidence of early African Homo actively hunting animals like gazelles and wildebeest 2 million years ago. Homo may have started cooking food around the same time.

Other foods have only become global staples in truly recent times. Potatoes and tomatoes are both from the Americas, so while they were domesticated thousands of years ago, nobody in Europe, Africa or Asia had even heard of them until the colonial period. Hence, it is slightly odd that The Lord of the Rings, which is supposed to be a lost British-European mythology, features hobbits eating poh-tay-toes. Turnips would be more in keeping.

Anyway, all of which is to say that this month we鈥檙e delving into humanity鈥檚 relationship with a unique group of foods: the citrus fruits, which include oranges, limes and kumquats.

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The history and prehistory of citrus is fascinating and mysterious. I鈥檓 going to cover it in three phases. To start, we鈥檒l look at how they were first domesticated. Second, we鈥檒l delve into their evolutionary history as revealed by genomics. And third, we鈥檒l try to put the hominins back into that evolutionary story.

The domestication of citrus

My source for a lot of this is a new book called by David J. Mabberley. As the name suggests, it鈥檚 the story of citrus fruits from their initial domestication to the present day. I was sent a review copy, but for various reasons the review didn鈥檛 happen, so I decided to peel it and share the best segments.

Let鈥檚 start with some basic terminology. We use the word 鈥渃itrus鈥� to describe fruits like oranges that have a fleshy centre with a sour, acidic taste, surrounded by a thick protective peel. But the word is also a taxonomic term. The genus Citrus refers to a group of flowering shrubs and trees. All citrus fruits grow on Citrus plants.

However, the relationships between the fruits and the species are knotty. That鈥檚 because Citrus species are very good at hybridising with one another. For instance, the familiar sweet oranges aren鈥檛 the fruit of a traditional species: instead, the plants they grow on are Citrus x sinensis, meaning they are hybrids of pomelos (Citrus maxima) and mandarin oranges (Citrus reticulata). As much as possible, I鈥檓 going to stick to just naming fruits, because otherwise we鈥檒l get bogged down.

Today, domesticated Citrus plants are grown in many warm countries. Florida is famous for its orange groves, and huge volumes are grown in Turkey, South Africa and Brazil, among others.

However, their origins lie in southern Asia 鈥� in the neighbourhood of China and the Himalayas. Mabberley says that the oldest confirmed record of Citrus cultivation is in a Chinese document called the Yu Gong, an account of China鈥檚 provinces as they were around 2000 BC, albeit compiled over a millennium later. It mentions oranges and pomelos being sent to the capital. presumably happened earlier, but we are very much in historical times.

Over the coming centuries, citrus fruits spread. Eastwards, tachibana oranges were known in Japan by the late 200s AD. To the west, citrons became the first citrus fruit known to Europeans. They seem to have been grown in Sumeria and Mesopotamia: Mabberley mentions a Persian fairy tale called The Girl of Narenj and Toranj, in which a girl dies and is resurrected from inside a fruit that may have been a citron. The association between citrus fruits and magic persists today, notably in Samantha Shannon鈥檚 fantasy The Priory of the Orange Tree.

There is a story that citrons were brought to Europe by Alexander the Great (356-323 BC) as part of his conquest of western Asia. However, Mabberley says it may be more complicated than that. The Greek philosopher Theophrastus, a contemporary of Alexander, wrote about fruits that sound a lot like citrons and didn鈥檛 treat them as novelties. Still, Alexander鈥檚 empire did increase east-west trade, so there may well be some truth in it.

Despite these uncertainties, the essence of the story is simple enough. Citrus was first domesticated in or around China, and from there the domesticated plants were carried throughout Eurasia and Africa, and eventually around the world.

Citrus evolution

However, wild Citrus plants are found beyond China. The Philippines has biasong and samuyao, while Citrus hystrix grows throughout South-East Asia. The genus has also reached Australasia. According to Mabberley, 鈥淣ew Caledonia has two or three native species, Papua New Guinea three, but Australia has six鈥�.

It has proved quite difficult to sort out the origins of all these Citrus plants. Because they are so good at cross-breeding, is tangled up in .

However, . Guohong Albert Wu at the US Department of Energy Joint Genome Institute in California and his colleagues compiled genomic and other data, and they concluded that all modern citrus plants are derived from 10 ancestral species. Between 8 and 6 million years ago, Citrus diversified rapidly in Asia, probably in and around the Himalayas. Later, some of them spread to Australasia, where a second evolutionary radiation occurred around 4 million years ago.

The physical archaeological record of Citrus is pretty sparse, but the story Wu鈥檚 team tells fits what we do know. They highlight that identified fossilised leaves from Lincang in China, which were identified as a new species dubbed Citrus linczangensis. The leaves were about 8 million years old.

Compared with the wild species, and . But that doesn鈥檛 mean the wild ones were inedible. On the contrary, many Citrus plants rely on animals to disperse their seeds. Monkeys, parrots and bush pigs are all known to eat citrus and poop out the seeds. So it seems reasonable to suppose that hominins ate them too, as they must have encountered them.

Hominins and citrus

You might think, hang on, didn鈥檛 you just say wild Citrus fruits are really sour? Well, yes, but humans quite like that. A 2020 review points out that , perhaps because acidic flavours are an indication of rot. Our species is one of the few exceptions.

Building on this, a 2022 review tracked . It argued that 鈥渢he common ancestor of monkeys and apes lost the ability to produce vitamin C鈥� between 61 and 74 million years ago, when a gene was deactivated. It may be that this happened because these early primates ate so much fruit that, unlike other mammals, they didn鈥檛 need to make their own .

However, later species that were more omnivorous were then at risk of scurvy, the lethal disorder that results from a lack of vitamin C. Eating citrus, and , would have protected them.

The review goes on to highlight evidence that modern great apes like or at least tolerate sour foods. Furthermore, a 1984 study found erosion on the teeth of a 1.7-million-year-old Homo habilis from Tanzania, .

The first hominins wouldn鈥檛 have encountered Citrus, because the first hominins lived only in Africa and Citrus hadn鈥檛 reached that continent when they first evolved around 7 million years ago. Instead, the first hominin-citrus encounter presumably came when some Homo erectus explored outside Africa, around 2 million years ago. H. erectus remains have been found on Java in Indonesia, implying they passed through what is now China.

Did H. erectus and other Asian hominins like the Denisovans eat citrus? The fact is that we have little evidence. However, a 2022 review of over the last 20,000 years found that they ate a huge variety of foods, and in one report this included citrus fruits.

Finally, I will float another possible use of citrus: honey hunting. To get honey out of bees鈥� nests, honey hunters need a way to pacify or deter the bees. This is often done with the smoke from fire, but of course hominins haven鈥檛 always been able to control fire. A 2015 study suggested , choosing plants that contained insect-repellent chemicals. In southern Asia, that could have included Citrus.

This last section is necessarily speculative. We don鈥檛 have hard evidence of hominins like H. erectus eating or otherwise interacting with Citrus, so all I鈥檝e done is present circumstantial evidence and try not to overuse the word 鈥減lausible鈥�.

But for what it鈥檚 worth, I do like the idea of an adventurous young H. erectus, somewhere in the foothills of the Himalayas, picking a wild citrus fruit from a tree. The thick peel would have been fiddly but wouldn鈥檛 have stopped them for long 鈥� until finally their eyes widened in shock and delight as the sour flesh hit their tongue for the first time.