3.1: A Brief History of Evolutionary Thought

The World Was Already Watching

The Greek philosopher Aristotle (384-322 BCE) was among the first to insist that the natural world could be understood through direct observation rather than divine explanation. He dissected animals, interviewed beekeepers and fishermen, and classified nearly 500 species based on their structure, reproduction, and behavior (Moore 1993). This begins to lay the foundation for nature as something to be studied, not simply accepted.

Aristotle also ranked living things in a hierarchy, placing animals above plants, live-bearing animals above egg-layers, and so on. Medieval scholars later expanded this into the Scala Naturae, or “Great Chain of Being,” a ladder of life with God at the top and minerals at the bottom, with humans placed just beneath the angels. This idea of “higher” and “lower” organisms shaped Western thinking for well over a thousand years, and it still lingers strongly today whenever someone describes an animal as “more evolved” than another (as we’ll see later, evolution doesn’t actually work that way).

Evolutionary thinking was never confined to Europe. The Islamic scholar Al-Jahiz (776-868 CE) built on Aristotle’s work in his seven-volume Book of Animals, describing over 350 species. Centuries before Darwin, Al-Jahiz wrote about animals competing for resources and mates, and about environments pushing organisms to develop new traits that could be passed to their offspring (Love 2020). He described a genuine “struggle for existence” shaping which animals survived (Masood 2009). These ideas traveled through Islamic scholarship into medieval European universities via translations of Aristotle (Lindberg 1992), a reminder that the road to Darwin ran through many cultures, not a straight line through Western Europe alone.

The Tools Science Needed First

Before anyone could explain how species change, scientists needed some basic tools. Francis Bacon (1561-1626) helped establish the scientific method: the idea that claims about nature should be tested against careful observation, not settled on authority alone. John Ray (1627-1705) offered one of the first working definitions of a species. Carl Linnaeus (1707-1778) gave the world binomial nomenclature, the two-part naming system (genus and species) still used today, along with a systematic way to organize the sheer diversity of life. And Georges-Louis Leclerc, Comte de Buffon (1707-1788), popularized comparative anatomy, comparing body structures across species, and was among the first prominent naturalists to suggest that species themselves might change over time.

Does the Earth Have Enough Time?

For evolution to happen gradually, the Earth needs to be old, really old. This became one of the fiercest debates in science. Georges Cuvier (1769-1832) proved that species that had once roamed the earth had gone extinct by studying fossils of animals with no living counterparts. He argued that species themselves never changed, but instead, catastrophic floods wiped out life in a region, which was then repopulated from elsewhere. This view, called catastrophism, fit neatly within a Bible-based timeline suggesting the Earth was only about 6,000 years old.

Geologists James Hutton (1726-1797) and Charles Lyell (1797-1875) argued the opposite: that the same slow, ordinary processes we see today—erosion, sedimentation, volcanic activity—have been shaping the Earth for an immense span of time, an idea known as uniformitarianism. Lyell’s guiding principle, “the present is the key to the past,” meant that ripples in ancient sandstone could be explained by watching ripples form in sands today, just multiplied across millions of years. Lyell became a close friend of Darwin’s, and his three-volume Principles of Geology (1830-1833) gave Darwin exactly the kind of “deep time” his theory of gradual change would eventually require.

A First (Wrong) Mechanism, and a Missing Piece

Jean-Baptiste Lamarck (1744-1829) deserved credit for a genuine breakthrough: he was the first scientist to propose an actual mechanism for how species change, and to recognize that the environment plays an active role in shaping organisms. His theory, the inheritance of acquired characteristics, held that traits acquired through use during an individual’s life could be passed on directly to that individual’s offspring.

Lamarck’s theory involved a three-step process. In step one, an animal experiences a radical change in its environment. In step two, the animal (either individual or species) responds with a new kind of behavior. In step three, that behavioral change results in morphological (meaning physical) changes to the animal, which are then successfully passed on to subsequent generations (Ward 2018, 8). Lamarck’s most famous example was the proposition that giraffes actively stretched their necks to reach leaves on tall trees to eat. Over their lifetimes, the continuation of this habit resulted in gradual lengthening of the neck. The

How giraffes actually ended up with long necks is a different story. In an environment where the food supply is higher off the ground, and perhaps less available to competing species, giraffes who happened to have slightly longer necks (due to random individual variation and genetic mutation) would be more likely to survive. These giraffes would then be able to reproduce, passing along the slight variation in neck length that would allow their offspring to do the same. Over time, individuals with longer necks would be overrepresented in the population, and neck lengths overall would increase among giraffes.

The Evolution of Long Necks in Giraffes

By Douglas R. Cavener, Huck Distinguished Chair in Evolutionary Genetics and Professor of Biology, Penn State

Everything in biology ultimately boils down to food and sex. To survive as an individual you need food. To survive as a species you need sex.

Not surprisingly then, the age-old question of why giraffes have long necks has centered around food and sex. After debating this question for the past 150 years, biologists still cannot agree on which of these two factors was the most important in the evolution of the giraffe’s neck. In the past three years, my colleagues and I have been trying to get to the bottom of this question.

Necks for sex

In the 19th century, biologists Charles Darwin and Jean Baptiste Lamarck both speculated that giraffes’ long necks helped them reach acacia leaves high up in the trees, though they likely weren’t observing actual giraffe behavior when they came up with this theory. Several decades later, when scientists started observing giraffes in Africa, a group of biologists came up with an alternative theory based on sex and reproduction.

These pioneering giraffe biologists noticed how male giraffes, standing side by side, used their long necks to swing their heads and club each other. The researchers called this behavior “neck-fighting” and guessed that it helped the giraffes prove their dominance over each other and woo mates. Males with the longest necks would win these contests and, in turn, boost their reproductive success. That favorability, the scientists predicted, drove the evolution of long necks.

Since its inception, the necks-for-sex sexual selection hypothesis has overshadowed Darwin’s and Lamarck’s necks-for-food hypothesis.

The necks-for-sex hypothesis predicts that males should have longer necks than females, since only males use them to fight, and indeed they do. But adult male giraffes are also about 30% to 50% larger than female giraffes. All of their body components are bigger. So my team wanted to find out if males have proportionally longer necks when accounting for their overall stature, comprised of their head, neck and forelegs.

Necks not for sex?

But it’s not easy to measure giraffe body proportions. For one, their necks grow disproportionately faster during the first six to eight years of their life. And in the wild, you can’t tell exactly how old an individual animal is. To get around these problems, we measured body proportions in captive Masai giraffes in North American zoos. Here, we knew the exact age of the giraffes and could then compare this data with the body proportions of wild giraffes that we knew confidently were older than 8 years.

To our surprise, we found that adult female giraffes have proportionally longer necks than males, which contradicts the necks-for-sex hypothesis. We also found that adult female giraffes have proportionally longer body trunks, while adult males have proportionally longer forelegs and thicker necks.

Giraffe babies don’t have any of these sex-specific body proportion differences. They only appear as giraffes are reaching adulthood.

Finding that female giraffes have proportionally both longer necks and longer body trunks led us to propose that females, and not males, drove the evolution of the giraffe’s long neck, and not for sex but for food and reproduction. Our theory is in agreement with Darwin and Lamarck that food was the major driver for the evolution of the giraffe’s neck, but with an emphasis on female reproductive success.

A shape to die for

Giraffes are notoriously picky eaters and browse on fresh leaves, flowers and seed pods. Female giraffes especially need enough to eat because they spend most of their adult lives either pregnant or providing milk to their calves.

Females tend to use their long necks to probe deep into bushes and trees to find the most nutritious food. By contrast, males tend to feed high in trees by fully extending their necks vertically. Females need proportionally longer trunks to grow calves that can be well over 6 feet tall at birth.

For males, I’d guess that their proportionally longer forelegs are an adaptation that allows them to mount females more easily during sex. While we found that their necks might not be as proportionally long as females’ necks are, they are thicker. That’s probably an adaptation that helps them win neck fights.

But giraffes’ necks aren’t their only long feature. They have very long legs, proportionally, which contribute to their height almost as much as their necks. Their long legs come at a considerable cost, though – particularly for male giraffes. A disproportionate fraction of their body mass is stacked on top of their spindly front legs, which can lead to injury and mobility issues in the long run.

Graham Mitchell, a prominent giraffe biologist, has called the giraffe body “a shape to die for.” In captivity, where staff can determine the cause of death, well over half of male giraffes die from foreleg problems, which shortens their lifespan by 25% compared with females. Very few female giraffes die from health issues related to their legs.

Giraffes’ height also means they can’t climb up steep slopes very well. My team’s research has shown that this limitation has likely stopped them from traveling across the escarpments of the Great Rift Valley in East Africa. But the mating advantage from being tall must outweigh these costs to their health and mobility.

This research isn’t ruling out the necks-for-sex theory entirely. The long neck likely does play a critical role in male neck-fighting and winning a mate. But our research suggests that male neck-fighting was probably a side benefit that came along with females getting better access to food.

In the future, my team will look into the genetic factors that led to the giraffe’s extraordinary stature and physique. We want to trace and reconstruct the evolutionary path they took to reach toward the skies.

Source: Cavener, D. (2024, June 5). Female giraffes drove the evolution of long giraffe necks in order to feed on the most nutritious leaves. The Conversation. https://theconversation.com/female-giraffes-drove-the-evolution-of-long-giraffe-necks-in-order-to-feed-on-the-most-nutritious-leaves-new-research-suggests-227029

Lamarck had the right instincts but the wrong mechanism. We know a bodybuilder’s children aren’t born muscular. Acquired traits simply don’t get passed on this way. Interestingly though, modern epigenetics has shown that some life experiences — chronic stress, poor nutrition, toxin exposure — really can leave chemical marks on gene expression that get passed to offspring. It’s a fascinating and fast-growing area of research we’ll return to later, but it works through very different mechanisms than Lamarck imagined.

The final piece came from someone who wasn’t a naturalist at all. Thomas Malthus (1766-1834), an English economist, published An Essay on the Principles of Population in 1798, arguing that populations naturally grow faster than their food supply, a concept still used today as carrying capacity. In Malthus’s England, this played out as poverty and overcrowding during the Industrial Revolution; in nature, Malthus argued, the same math applies to every species. Some individuals will always be born who don’t survive to reproduce. Malthus never intended his idea for biology, but it became the missing piece Darwin needed.

By the 1850s, then, science had assembled most of the picture: an ancient Earth (Hutton, Lyell), evidence that species really do change and go extinct (Cuvier, Buffon), and a reason why not every individual survives (Malthus). What remained unanswered was the question this chapter is really about: out of all the individual variation in a population, what determines which traits get passed on and which don’t? That answer would come from one of the greatest synthesizers and systems thinkers of his time, a naturalist named Charles Darwin.