Life never would have evolved on Earth without death. It may seem counterintuitive, but without the natural cycle of life and death and the food chain it created, life never would have evolved from simple, single-celled organisms to the complex creatures that roam our planet today. In a new episode of the documentary series "One Strange Rock," which airs tonight (April 23) on the National Geographic Channel, astronauts explain how death drove the evolution of life on Earth.

"One Strange Rock," which is hosted by Will Smith and narrated by astronauts, tells the story of life on Earth, starting with the violent collisions and "lucky" coincidences billions of years ago that helped make the planet habitable. In last week's episode, retired NASA astronaut Mae Jemison explained how the first living organisms emerged from a few simple ingredients.

The next episode, titled "Survival," shows how death allowed those early life-forms to evolve, bringing biodiversity to a world that was once dominated by microbes. Understanding how this happened on Earth could provide scientists with clues about what extraterrestrial life may look like, the episode explains. [7 Theories on the Origin of Life]

Kurile Lake in in Kamchatka, Russia is a breeding ground for six species of Pacific salmon, which migrate from the ocean to this lake to reproduce. The fish breed only once and die immediately afterward, leaving behind thousands of offspring. This phenomenon is called semelparity, or "big bang reproduction."
Kurile Lake in in Kamchatka, Russia is a breeding ground for six species of Pacific salmon, which migrate from the ocean to this lake to reproduce. The fish breed only once and die immediately afterward, leaving behind thousands of offspring. This phenomenon is called semelparity, or "big bang reproduction."
Credit: National Geographic

"Every creature on Earth evolved its body shape to maximize chances of getting a meal without being eaten," Smith says in the episode. This concept of "eat or be eaten" has been the rule of life ever since the very first life-forms appeared on Earth almost 4 billion years ago. Back then, microbes were the only living things on Earth. It was totally normal for microbes to eat each other, but one exceptional microbial meal may have been responsible for creating more complex life.

Researchers believe that evolution really began when one particular single-celled microbe (known as an archaeon) ingested a bacterium. Instead of being digested, this meal survived inside the archaeon and became a permanent part of its new host. Together, they formed the first complex (or eukaryotic) cell, with the bacterium functioning as a mitochondrion, or the cell's powerhouse.

"This remarkable microbe had a superpower: It generated energy using oxygen, allowing it to extract 15 times more energy from its food," Jemison says in the episode. "This is the moment when life as we know it really began." Thanks to this new complex cell — and especially the mitochondria — life managed to evolve into bigger and more diverse creatures.

If and when humans manage to find extraterrestrial life, it will probably look more like those single-celled organisms than any two-legged humanoids, the astronauts explain in the show.

You can watch the new episode on the National Geographic Channel tonight (April 23) at 10 p.m. EDT/PDT (9 p.m. CDT).

Email Hanneke Weitering at hweitering@space.com or follow her @hannekescience. Follow us @Spacedotcom, Facebookand Google+. Original article on Space.com.