The Dispersal of Ancient and Modern Apples by Humans and Other Megafauna

Crop domestication often involves selection for larger fruits. In some crops, humans took plant species with relatively small fruits and, over many generations of artificial selection, developed a plant with much larger fruits. Consider giant pumpkins as an extreme example. Yet in the case of apples, relatively large fruits already existed in the wild. Producing larger apples happened quickly and, perhaps even, unconsciously. Apples were practically primed for domestication, and as Robert Spengler explains in a paper published last year in Frontiers in Plant Science, looking back in time at the origins of the apple genus, Malus, can help us understand how the apple we know and love today came to be.

Apples are members of the rose family (Rosaceae), a plant family that today consists of nearly 5000 species. According to the fossil record, plants in the rose family were found in large numbers across North America as early as the Eocene (56 – 33.9 million years ago). They were present in Eurasia at this time as well, but Spengler notes, “there is a much clearer fossil record for Rosaceae fruits and seeds in Europe and Asia during the Miocene and Pliocene (20 – 2.6 million years ago).” Around 14 million years ago, larger fruits and tree-form growth habits evolved in Rosaceae subfamilies, giving rise to the genera Malus and Pyrus (apples and pears). Small, Rosaceae fruits were typically dispersed by birds, but as Sprengler writes, “it seems likely that the large fruits [in Malus and Pyrus] were a response to faunal dispersers of the late Miocene through the Pliocene of Eurasia.” Larger animals were being recruited for seed dispersal in a changing landscape.

Glacial advances and retreats during the Pleistocene (2.6 million – 11,700 years ago) brought even more changes. Plants with effective, long distance seed dispersal were favored because they were able to move into glacial refugium during glacial advances. Even today, these glacial refugium are considered genetic hot spots for Malus, and could be useful for future apple breeding. As the Pleistocene came to a close, many megafauna were going extinct. This continued into the Holocene. Large-fruited apple species lost their primary seed dispersers, and their ranges became even more contracted.

Humans have had an extensive relationship with apples, which began long before domestication. Foraging for apples was common, and seeds were certainly spread that way (perhaps even intentionally). Favorable growing conditions were also created when forests were cleared and old fields were left fallow. Apple trees are early successional species that easily colonize open landscapes, gaps in forests, and forest edges, so human activity that would have created such conditions “could have greatly promoted the spread and success of wild Malus spp. trees during the Holocene.”

The earliest evidence we have of apple domestication (in which “people were intentionally breeding and directing reproduction”) occurred around 3000 years ago in the Tian Shan Mountains of Kazakhstan, where Malus sieversii – a species that is now facing extinction – was being cultivated. This species was later brought into contact with other apple species, a few of which were also being cultivated, including M. orientalis, M. sylvestris, and M. baccata. These species easily hybridized, giving us the modern, domesticated apple, M. domestica. As Spengler writes, “the driving force of apple domestication appears to have been the trans-Eurasian crop exchange, or the movement of plants along the Silk Road.” Continued cultivation and further hybridization among M. domestica cultivars over the past 2000 years has resulted in thousands of different apple varieties.

The unique thing about domesticated apples is that their traits are not fixed in the same way that traits of other domesticated crops are. Growing an apple from seed will result in a very different apple than the apple from which the seed came. Apple traits instead have to be maintained through cloning, which is accomplished mainly through cuttings and grafting. Apples hybridize with other apple species so readily that most apple trees found in the wild are hybrids between wild and cultivated populations.

Spengler considers the study of apple domestication to be “an important critique of plant domestication studies broadly, illustrating that there is not a one-size fits-all model for plant domestication.” The “key” for understanding apple domestication “rests in figuring out the evolutionary driver for large fruits in the wild – seed dispersal through megafaunal mammals – and the process of evolution for these large fruits – hybridization.” He notes that “domestication studies often ignore evolutionary processes leading up to human cultivation,” which, in the case of apples, involves “hybridization events in the wild” that led to the evolution of large fruits “selected for through the success in recruiting large megafaunal mammals as seed disperses.” Many of those mammals went extinct, but humans eventually assumed the role, selecting and propagating “large-fruiting hybrids through cloning and grafting – creating our modern apple.”

Excerpt from Fruit from the Sands by Robert N. Spengler:

Indeed, the relationship between apples and people is close and complex, spanning at least five millennia. The story of the apple begins along the Silk Road… In recent years genetic studies have resolved much of the debate over these origins. Nevertheless, the ancestry of the apple is highly complex. Cloning, inbreeding, and reproduction between species have created a genealogy that looks more like a spider’s web than a family tree. To growers, the beauty of the apple lies not in its rosy skin but in its genetic variability and plasticity, its ability to cross with other species of Malus and other distant lines of M. domestica, and the ease with which it can be grafted onto different rootstocks and cloned.

See Also: Science Daily – Exploring the Origins of the Apple

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Interested in learning more about how plants get around? Check out the first issue of our new zine Dispersal Stories.

Book Review: Fruit from the Sands

“By dispensing plants and animals all around the world, humans have shaped global cuisines and agricultural practices. One of the most fascinating and least-discussed episodes in this process took place along the Silk Road.” — Fruit from the Sands by Robert N. Spengler III

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My understanding of the origins of agriculture and the early years of crop domestication are cursory at best. The education I received was mostly concerned with the Fertile Crescent, as well as crops domesticated by early Americans. The Silk Road, as I understood it, was the route or routes used to move goods across Asia and into eastern Europe well after the domestication of many of the crops we know today. Other than the fact that several important crops originate there, little was ever taught to me about Central Asia and its deep connection to agriculture and crop domestication. I suppose that’s why when I picked up Fruit from the Sands by Robert N. Spengler III, published last year by University of California Press, I wasn’t entirely prepared for what I was about to read.

It wasn’t until I read a few academic reviews of Fruit from the Sands that I really started to understand. Spengler’s book is groundbreaking, and much of the research he presents is relatively new. The people of Central Asia played a monumental role in discovering and developing much of the food we grow today and some of the techniques we use to grow them, and a more complete story is finally coming to light thanks to the work of archaeologists like Spengler, as well as advances in technology that help us make sense of their findings.

This long history with agricultural development can still be seen today in the markets of Central Asia, which are loaded with countless varieties of fruits, grains, and nuts, many of which are unique to the area. Yet this abundance is also at risk. Crop varieties are being lost at an alarming rate with the expansion of industrial agriculture and the reliance on a small selection of cultivars. With that comes the loss of local agricultural knowledge. Yet, with climate change looming, diversity in agriculture is increasingly important and one of the tools necessary for maintaining an abundant and reliable food supply. Unveiling a thorough history of our species’ agricultural roots will not only give us an understanding as to how we got here, but will also help us learn from past successes and failures. Hence, the work that Spengler and others in the field of archaeobotany are doing is crucial.

To set the stage for a discussion of “the Silk Road origins of the foods we eat,” Spengler offers his definition of the Silk Road. The term is misleading because there isn’t (and never was) a single road, and the goods, which were transported in all directions across Asia, included much more than silk. In fact, some of what was transported wasn’t a good at all, but knowledge, culture, and religion. In Spengler’s words, “The Silk Road … is better thought of as a dynamic cultural phenomenon, marked by increased mobility and interconnectivity in Eurasia, which linked far-flung cultures….This network of exchange, which placed Central Asia at the center of the ancient world, looked more like the spokes of a wheel than a straight road.” Spengler also sees the origins of the Silk Road going back at least five thousand years, much earlier than many might expect.

Most of Spengler’s book is organized into chapters discussing a single crop or group of crops, beginning with grains (millet, rice, barley, wheat) then moving on to fruits, nuts, and vegetables before ending with spices, oils, and teas. Each chapter compiles massive amounts of research that can be a bit overwhelming to take in all at once. Luckily each section includes a short summary, which nicely distills the information down into something more digestible.

Spengler’s chapter on broomcorn millet (Panicum miliaceum) is particularly powerful. While in today’s world millet has largely “been reduced to a children’s breakfast food in Russia and bird food in Western Europe and America,” it was “arguably the most influential crop of the ancient world.” Originally domesticated in East Asia, “it passed along the mountain foothills of Central Asia and into Europe by the second millennium BC.” It is a high-yielding crop adapted to hot, dry conditions that, with the development of summer irrigation, could be grown year-round. These and other appealing qualities have led to an increase in the popularity of millet, so much so that 2023 will be the International Year of Millets.

The “poster child” for Spengler’s book may very well be the apple. Popular the world over, the modern apple began its journey in Central Asia. As Spengler writes, “the true ancestor of the modern apple is Malus sieversii,” and “remnant populations of wild apple trees survive in southeastern Kazakhstan today.” As the trees were brought westward, they hybridized with other wild apple species, bringing rise to the incredible diversity of apple cultivars we know today. Sadly though, most of us are only familiar with the small handful of common varieties found at our local supermarkets.

Of course, as Spengler says, “No discussion of plants on the Silk Road would be complete without the inclusion of tea (Camellia sinensis),” a topic that could produce volumes on its own. Despite the brevity of the section on tea, Spengler has some interesting things to say. One in particular involves the transport of tea to Tibet in the seventh century, where “an unquenchable thirst for tea” had developed. But the journey there was long and difficult. Fermented and oxidized tea leaves traveled best. Along the way, “the leaves were exposed to extreme cold as well as hot and humid temperatures in the lowlands, and all the time they were jostled on the backs of sweaty horses and mules.” This, however, only improved the tea, as teas exposed to such conditions “became a highly sought-after commodity among the elites.”

“In Central Asia, Mongolia, and Tibet, tea leaves were oxidized, dried, and compressed into hard bricks from which chunks could be broken off and immersed in water.” – Robert N. Spengler III in Fruit from the Sands (photo via wikimedia commons)

As dense as this book is, it’s also quite approachable. The information presented in each of the chapters is thorough enough to be textbook material, but Spengler does such a nice job summing up the main points, that there are plenty of great takeaways for the casual reader. For those wanting a deeper dive into the history of our food (which in many ways is the history of us), Spengler’s book is an excellent starting point.

More Reviews of Fruit from the Sands:

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Podcast Review: Gastropod

I am a voracious consumer of podcasts and have a long list that I regularly listen to. Despite being unable to get through all of them in a reasonable amount of time, I am still continually on the lookout for more. I am particularly interested in science or educational podcasts – something that I can listen to for an hour or so and learn new things about the world, whether it be breaking news or historical facts.

This year a new podcast was born – a podcast exploring the science and history of food.  It is called Gastropod, and it has quickly found its way into my regular rotation of podcast consumption. It wasn’t a difficult climb either, as the general theme of the podcast is something that fascinates me and the hosts do a top-notch job presenting the information and telling the stories.

gastropod

Gastropod is hosted by Cynthia Graber and Nicola Twilley, each of whom have impressive backgrounds in researching and reporting on science, technology, food, and other topics for a variety of outlets both large and small. Among numerous other projects, Nicola has a blog called Edible Geography and Cynthia contributes regularly to Scientific American’s 60 Second Science podcast. Gastropod just happens to be their latest endeavor, and it is a welcome one.

Full length episodes of Gastropod are released once a month, with “snack-sized interludes” called Bites released in between to tide listeners over until the next helping. Since Gastropod is in its infancy (the first episode was released in September 2014), catching up on past episodes is simple. An afternoon of binge listening will do it.

Topics covered so far in full length episodes include the history and evolution of cutlery (which involves a taste test using spoons made of various metals), a discussion with Dan Barber about his book The Third Plate, an exploration of the emerging “microbe revolution” in agriculture (which piggybacks on an article that Cynthia wrote for NOVANext and which I reviewed back in July), and the rising popularity of kelp (“the new kale”) and the growth of seaweed farms. Bite-sized episodes have discussed things like modern day domestication of wild plants, underused American seafood resources, a meal replacement drink called Soylent, the expansive yet underappreciated (and disappearing) diversity of apples, and subnatural foods (smoked pigeon, anyone?).

So far every episode has been great, but if I had to pick a favorite, the interview with Dan Barber really stands out. His discussion of “ecosystem cuisines” – which moves beyond the farm-to-table movement – was new to me but seems like an important idea and one that I would like to see play a pivotal role in the development of science-based sustainable agriculture.

Gastropod is a young but promising podcast, and I look forward to many more captivating episodes in 2015 and beyond. Learn more about Gastropod and its hosts here.

Do you have a favorite podcast, science-themed or otherwise? Share it in the comments section below.

Apples and Genetic Bottlenecks

This is the eleventh in a series of posts reviewing the 17 articles found in the October 2014 Special Issue of American Journal of Botany, Speaking of Food: Connecting Basic and Applied Science.

Genetic Diversity in Malus x domestica (Rosaceae) through Time in Response to Domestication by Briana L. Gross, Adam D. Henk, Christopher M. Richards, Gannara Fazio, and Gayle M. Volk

Domestication is a selection process. Plants with desirable traits are selected (consciously or unconsciously) and removed from the larger population to be grown out and selected from again. Over time, this series of selections results in a cultivated variety that differs substantially from the larger, origin population. This process, while yielding crop varieties that feed a growing population of humans, also results in a series of genetic bottlenecks, meaning they experience a reduction in genetic variation compared to their wild relatives.

There are two points were bottlenecks occur in the domestication process. The first takes place “during the initial domestication event as a subset of the wild population is brought into a cultivated setting.” This is called a “domestication bottleneck.” The second, known as an “improvement bottleneck,” happens when “modern, elite cultivars are selected from the broad variety of landraces [locally adapted varieties]” that were developed during the original domestication event. This stepwise reduction in genetic diversity “limits the options of plant breeders, even as they face the need to increase crop productivity and sustainability” in today’s changing climate.

Most of what we know about genetic bottlenecks during domestication is derived from studies of annual fruit and grain crops. However, “non-grain crops, and perennials in particular, respond to domestication or are domesticated in ways that are fundamentally different.” For this reason, the authors investigated genetic bottlenecks in apple (Malus x domestica), “one of the most widely distributed perennial fruit crops.” They then compared what they learned to other published studies of annual and perennial fruit crops in order to gain more insight into how genetic diversity is affected in these types of crops during domestication.

The common apple was domesticated in central Asia around 4,000 years ago and is a hybrid of at least three species: Malus sieversii, Malus orientalis, and Malus sylvestris. Today, apples are grown throughout the world, and there are more than 7,500 known cultivars with new cultivars being released regularly. Despite this impressive diversity, just fifteen cultivars make up 90% of apple production in the U.S. The authors of this study analyzed DNA from 11 of the 15 major cultivars as well as DNA from the three main wild progenitor species.

Malus x domestica 'Gala' - One of the top 15 apple varieties produced in the U.S. (photo credit: wikimedia commons)

Malus x domestica ‘Gala’ – One of the top 15 apple varieties produced in the U.S. (photo credit: wikimedia commons)

Perennial fruit crops typically experience “mild genetic bottlenecks” compared to annual fruit crops, and the authors confirmed this to be the case with domesticated apples, finding “no significant reduction in genetic diversity through time across the last eight centuries.” Because apple cultivars are maintained by clonal propagation, they can often be traced back to when they were originally developed, making bottlenecks easier to observe. The authors found that “the most recently developed or described cultivars of apples show little to no reduction in genetic diversity compared with the most ancient cultivars.” Cultivars developed since the 1950’s show increased diversity, which may partly be the result of plant breeders introducing genes from another wild species, Malus floribunda.

After a review of the literature, the authors found that apples have retained the highest amount of genetic diversity through the domestication process compared to other fruits, both annual and perennial. More studies are needed in order to confirm the accuracy and extent of these findings; however, the unique story of apple domestication may help explain why it has been “particularly prone to retaining diversity through time.” First, it was widely distributed across Eurasia during its early days of domestication. Second, it experienced “admixture with cultivars” as it expanded its range. For example, after being introduced to North America, it became naturalized, resulting in gene flow occurring between naturalized individuals and cultivated varieties. Offspring of these populations (“chance seedlings”), were then selected, cloned, and became named cultivars.

Despite the mild genetic bottleneck observed in apples, the authors warned that a “dependence on a small number of cultivars” for the majority of U.S. apple production may be resulting in some loss of genetic variation. Relying on so few cultivars may leave apple production vulnerable to pests, diseases, and climate change. “Careful management” is advised as “the continued genetic resilience of the crop is dependent on the genetic diversity of cultivars that are present in living and cryopreserved collections around the world.”

Malus sylvestris (common crabapple) - One of the three main players involved in the apple domestication story (photo credit: www.eol.org)

Blossoms of Malus sylvestris (common crabapple) – One of three main species involved in the history of apple domestication (photo credit: www.eol.org)