In a patch of meadow tucked away in New York, a small experiment played out that turned one of biology’s most enduring narratives on its head. The question driving this study: what happens when luck — not strength, smarts, or savvy — becomes the architect of evolution?

Matthew Zipple, an evolutionary biologist at Cornell University, and his colleagues decided to test this idea by engineering a utopia — at least for mice. Each mouse in their study was genetically identical. They shared the same environment, the same early-life resources, and identical access to food and shelter. And yet, by the experiment’s end, the outcomes were anything but equal.

A World of Equal Beginnings

In this carefully controlled experiment, Zipple and his team released 26 young male mice and their mothers into outdoor enclosures. The design mirrored natural habitats, with one crucial twist: it eliminated any genetic or environmental head starts. The mice faced the same food availability, identical shelter, and identical opportunities — an evolutionary blank slate. Over 46 days, the researchers watched what happened.

“We wanted to know,” Zipple told NPR, “if we create a society where everyone starts out with the same genetics, has access to the same resources in the same environment… do we see that inequality developing?”

The answer came quickly. Male mice, subject to intense competition for territory and resources, began diverging early. A single scuffle over food could tip the scales, giving one mouse a slight edge in weight and strength now that it had access to more resources. That edge snowballed, enabling it to win future conflicts. By the experiment’s end, these early “lucky” mice dominated territories and interacted with five times as many females as their less fortunate counterparts.

The same wasn’t true for female mice, whose less competitive lives left them on relatively equal footing. “They still have good days and bad days,” noted Michael Sheehan, the study’s senior author, “but those don’t rack up over time in quite the same way.”

Luck’s Role in Evolution

The findings underscore a growing realization among scientists: randomness, long overshadowed by Darwinian competition, plays an important central role in evolution.

Yes, randomness has always been part of natural selection, stemming primarily from the genetic mutations that fuel evolution. Mutations arise unpredictably due to errors in DNA replication, environmental factors like radiation, or even biological mechanisms within cells. These mutations, though random in origin, are filtered by natural selection, which evaluates their impact on an organism’s survival and reproductive success. Beneficial mutations may spread through a population over generations, while harmful ones are typically eliminated. This randomness operates at the molecular level and is the starting point for evolutionary change.

In contrast, the Cornell study highlights a different kind of randomness — one that plays out even when the genetic and environmental playing fields have been leveled. By working with genetically identical mice in identical environments, researchers removed the randomness inherent to mutation and environmental variation. What remained was the randomness of life events: chance encounters, early scuffles, or fortuitous access to resources. These “micro-contingencies” were amplified by competition, creating stark inequalities among individuals who started with identical genetic and environmental advantages.

While natural selection undeniably shapes species over time, it often works hand-in-hand with chance events — small contingencies that send ripples through populations.

These compounding effects are fundamentally different from the probabilistic processes of mutation or genetic drift because they arise from interactions between individuals in a competitive context. While inherent randomness drives the raw variation upon which natural selection acts, the randomness unveiled by this study shows how life’s twists of fate can shape evolutionary outcomes even in a world of equals.

Replaying Evolution’s Tape

In his 1989 book Wonderful Life, Stephen Jay Gould provides a broader philosophical lens for interpreting these findings. Gould imagines “replaying the tape of life,” resetting evolution to its origins and allowing it to unfold again. His hypothesis: the outcomes would be entirely different, with life forms unrecognizable to us today. Gould attributes this variability to the inherent randomness of evolution, where small chance events early in life’s history profoundly influence its trajectory.

At the heart of Wonderful Life is Gould’s examination of the Burgess Shale, a 508-million-year-old fossil deposit discovered in the Canadian Rockies. The Burgess Shale is famous for its exceptional preservation of Cambrian-period organisms, many of which were soft-bodied and would typically decay before fossilization. These fossils provided an unprecedented glimpse into early animal life and revealed a stunning diversity of body plans — many of which have no modern counterparts.

Gould used the Burgess Shale as a springboard to argue that evolution is not a straightforward, deterministic process leading inevitably to the life forms we see today. Instead, he posited, it is shaped by randomness and contingency — events so dependent on specific circumstances that replaying Earth’s evolutionary history would likely produce an entirely different world. For example, Gould highlighted how many Burgess Shale species, which were once considered evolutionary “experiments,” ultimately went extinct. Meanwhile, others with no apparent superiority survived and gave rise to modern animal groups. This, he argued, underscores how small initial differences or chance events can cascade into dramatically divergent outcomes over time.

If we were to rewind time to the Cambrian period and let history unfold again, Gould believed, the path of evolution would be so contingent on chance events — such as which organisms survive a mass extinction or which mutations arise — that entirely new forms of life would emerge. Humans, he suggested, are not the inevitable pinnacle of evolution but rather one product of a long series of random events.

Wonderful Life: The Burgess Shale and the Nature of History

• Gould, Stephen Jay (Author)
• English (Publication Language)
• 352 Pages – 09/17/1990 (Publication Date) – W. W. Norton & Company (Publisher)

$18.14

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The Neutral Theory

Gould’s argument clashed with traditional views of evolution as a process guided primarily by natural selection. Instead, he highlighted the interplay between randomness and necessity, aligning his ideas with other thinkers who emphasize the probabilistic and contingent nature of evolutionary processes, such as Motoo Kimura’s neutral theory of evolution.

Motoo Kimura, who introduced neutral theory in 1968, argued that most genetic variations are neither helpful nor harmful. Instead, they persist or vanish based on probability alone. For instance, a mutation might spread simply because its carrier happened to avoid predators or find a mate.

Neutral theory also explains why evolution doesn’t always reward advantageous traits. A beneficial mutation — say, white fur in a snowy habitat — might seem destined to dominate a population. But it could disappear entirely if its initial carrier dies prematurely or fails to reproduce. This probabilistic nature of evolution shows that even advantageous traits are sometimes at the mercy of chance.

The role of luck becomes even more pronounced in small populations. Take the Northern elephant seal. In the late 19th century, overhunting reduced the species to just 30 individuals. Today, their population has rebounded to tens of thousands, but their genetic diversity remains alarmingly low. Mutations that could have improved immunity or swimming efficiency were lost along with the individuals who carried them.

Similarly, in the human lineage, bottlenecks have left enduring marks. After a devastating typhoon struck the Pacific island of Pingelap in the 18th century, just a handful of survivors repopulated the island. Among them was a carrier of a rare mutation causing extreme color blindness. Today, 10% of Pingelapese carry the mutation, compared to just 1 in 30,000 people globally.

Chance Meets Necessity

The Cornell mouse experiment, like many before it, challenges a tidy view of evolution as solely “survival of the fittest.” It turns out that luck can amplify small differences into life-altering advantages. As Robin Snyder, a theoretical ecologist uninvolved in the study, puts it: “As you dial up competition, it’s amplifying the effect of these small moments of coin flips.”

This understanding carries broader implications. Beyond science, it echoes in human society, where early strokes of fortune — or misfortune — can compound over a lifetime. It also forces us to grapple with moral questions about fairness and success. “What these results really emphasize,” said Zipple, “is that just because an individual ends up with less resources than others, that doesn’t mean that they’re to blame for that outcome.”

As science delves deeper into the interplay between chance and necessity, one thing becomes clear: luck isn’t an anomaly in evolution. It’s a core feature. Replaying life’s tape — even under identical conditions — would likely yield a world profoundly different from the one we know.

The new findings appeared in the journal Science.