Life on Earth, Part 2: Complex Life

For complex life to arise, it's all about stability, oxygen, and a little luck.

Earth formed about 4.54 billion years ago, and around 4 billion years ago, we find the first evidence of simple lifeforms. These single-celled organisms have dominated the planet for most of its history. It wasn’t until much later, during the Cambrian Explosion, which started about half-a-billion years ago and lasted tens-of-millions of years, that complex life literally exploded on Earth. During this time, practically all major animal phyla started appearing in the fossil record. This complex life was aquatic, rather small, and completely alien looking, at least to us humans. One example of a Cambrian creature is the trilobite, a spectacularly successful animal that existed in oceans for almost 270 million years, with over 22,000 species having been discovered.

Do you want to see evidence of the Cambrian Explosion in real life? If you ever find yourself hiking or rafting the Grand Canyon, it is worth checking into a set of geologic formations known as the Tonto Group, including the Tapeats Sandstone, Bright Angel Shale, and Muav Limestone, where fossils from the Cambrian Explosion, including trilobites, are plainly evident. But don’t expect to see dinosaur fossils, since the canyon rocks are older than the oldest known dinosaurs. [More info is available from the National Park Service.]

Here is a full list of ‘Cambrian Parks’ in the United States:

• Chesapeake and Ohio Canal National Historical Park, West Virginia, Washington D.C., and Maryland

• Death Valley National Park, California, Nevada

• Grand Canyon National Park, Arizona

• Great Basin National Park, Nevada

• Great Smoky Mountains National Park, Tennessee and North Carolina

• Harpers Ferry National Historical Park, Maryland, Virginia, and West Virginia

• Saint Croix National Scenic Riverway, Minnesota and Wisconsin

• Yellowstone National Park, Idaho, Montana, Wyoming

• Yukon-Charley Rivers National Park, Alaska

And in the Canadian Rocky Mountains of British Columbia, a number of hiking trails in the Burgess Shale, an exposed fossil-bearing deposit, provide probably the best outdoor opportunities to see Cambrian fossils up-close. [More info is available from Parks Canada.]

A view into the Grand Canyon, including the deeper layers where Cambrian fossils lay frozen in time [Wikipedia, CC BY 3.0, Lennart Sikkema]

Photo courtesy of Parks Canada. “High in the mountains of Yoho and Kootenay national park, the Burgess Shale fossils are the oldest evidence of complex life on Earth. With fine details so well preserved, they show a diverse marine ecosystem that existed long before the dinosaurs.” Note the fossil in the foreground.

An exceptionally well-preserved trilobite from the Burgess Shale, British Columbia, Canada. Notice the soft-tissue antenna. [Wikipedia, CC BY 2.5, Smith609]

Alright, so we know that at some point, around the Cambrian Explosion, complex life appeared on Earth. And to this day, complex life remains on Earth. To find out why, let’s dive in a bit further.

There are two big Paleoclimate questions when it comes to complex life on Earth.

1. What was the trigger mechanism for complex life to evolve?

2. What climatic conditions were needed to sustain complex life?

Starting with question #1, it turns out that oxygen was probably the primary ingredient needed to fuel a rapid increase in the diversity of complex life. A recent research paper, titled Possible links between extreme oxygen perturbations and the Cambrian radiation of animals, states that “fluctuations in oxygen availability in the shallow marine realm exerted a primary control on the timing and tempo of biodiversity radiations at a crucial phase in the early history of animal life.” In other words, during the Cambrian Explosion, spikes and dips in oxygen in shallow ocean areas correlated with increases or decreases in biodiversity. This certainly aligns with modern observations of ocean life where well-oxygenated waters are generally associated with larger body sizes and a higher diversity of life. Outside the ocean, mammals require large amounts of oxygen just to move and metabolize food. In short, many lines of evidence show that oxygen is very important for complex life.

Moving onto question #2, how has complex life, including all of us, survived a half-billion years since the Cambrian Explosion? That’s a long time for nothing to go terribly wrong! Here is a list - probably not exhaustive - of why complex life continues to survive on Earth:

1. Oxygen availability: The early-Earth had very little oxygen in the oceans or atmosphere, but this changed over time, and Earth has generally maintained high oxygen availability needed for complex life.

2. A long period of temperature stability: Earth temperatures have tended to exist within a very narrow and manageable range, not exceeding 100°C or falling below 0°C for long stretches of time.

3. A long period of water availability: Amazingly, Earth has had liquid water for 4 billion years, a prolonged period of wetness that must be rather rare in the universe.

4. A low asteroid-impact rate: A theory for why Earth has a low asteroid-impact rate is that Jupiter is the “vacuum cleaner of the solar system”, sucking in wayward asteroids with its gravity. However, counter to this point, Jupiter’s gravity also nudges some asteroids toward the Sun, where they have a greater possibility of eventually colliding with Earth. Whatever the case, limited asteroid encounters on Earth has resulted in a stable environment for sustaining complex life. One notable exception occurred sixty-five million years ago when most dinosaur species and >75% of all land and sea animals were suddenly wiped out by a massive asteroid impact. The resulting Chicxulub Crater is still evident today, buried underneath the Yucatán Peninsula in Mexico.

5. Favorable perturbations: Another term for “favorable perturbations” is “luck”. For example, the extinction of dinosaurs allowed mammals to thrive, and eventually humans evolved. We are now a powerhouse species, in part because of a large asteroid impact.

6. Location in galaxy: Earth hangs out in a nice corner of the Milky Way Galaxy. Beyond asteroids, we have avoided gamma ray bursts, supernovae, black holes (the Milky Way has one in the center), and more.

7. Version of Earth in the multiverse: Are there other versions of Earth that are less habitable in parallel universes? Who knows! But people keep talking about the multiverse, and a number of superhero movies have been based on it, including Doctor Strange in the Multiverse of Madness. I figured I would give the multiverse a shout-out, but I really know very little about it, and I suspect it’s partly an urban legend at this point. On the other hand, maybe String Theory says otherwise.

Strange Tales #110 (July 1963). In Marvel Comics, Doctor Strange serves as a primary protector of life on Earth against magical and mystical threats, including from the multiverse. [Wikipedia, Fair Use]

To sum it all up, the emergence of complex life appears to require adequate amounts of oxygen, and the survival of complex life requires long periods of climatic stability. I once overheard someone say that ‘stability’ could be one of the rarest things in the universe. The fact that Earth has been so stable for so long has also allowed complex life to evolve into intelligent life. How lucky! We really do have a very nice planet, don’t we?

One of my favorite intelligent lifeforms is the octopus [Wikipedia, CC BY-SA 2.0]. If you haven’t seen it, I strongly recommend watching the documentary film My Octopus Teacher, where filmmaker Craig Foster forges a relationship with a wild common octopus in a South African kelp forest.

When not eating tasty cucumber slices, the African grey parrot is especially good at language. They have been reported to use known words to create new names for unknown objects, such as "banerry" ("banana" + "cherry") for "apple”. The last words of one famous African grey parrot, named Alex, were “You be good, I love you. See you tomorrow.” [Wikipedia, CC0]

Thanks for reading, and please consider supporting this newsletter, it’s actually quite cheap, at least in this version of the multiverse.

Sincerely, 

TRJ, PaleoClimate Scientist