Life: this mysterious organization of responsive, reproducing systems growing more complex over time. We’re talking about wildly complex organizations- organisms! Yet, here we are, life on Earth just chugging along for four billion years like it’s no big deal. Maybe it isn’t a big deal. Maybe life is the natural evolution of what matter does as it gets more complex.
To get to the point we’re at now took some amazing evolutionary breakthroughs. So here in Part I are the first five of 10 breakthroughs in evolution to illustrate how far we’ve come.
1. Life begins 3.8 billion years ago
About a billion years after the Earth was formed, life emerged! In geologic time, life flew out of the starting blocks in nothing flat. Theories abound about how it all started. The speed with which life emerged suggests that it springs up rather easily under the right conditions. What the conditions were in the early years of our planet are not known for certain. What we do know is the first living things had all the ingredients for a long, successful, wild ride on Earth.
The first life on Earth was a simple, sturdy, single-celled organism called a prokaryote. There wasn’t much to these tiny, ocean-going creatures. Prokaryotes have only a cell wall, a cell membrane, a tiny loop of genetic material, and some inner goo. They don’t have a nucleus or any fancy organelles. But they do have flagella, little tails so they can swim, and some outer velcro-like material so they can stick around once they get where they’re going. Life definitely evolved the most basic model first.
The basic model was incredibly successful. Prokaryotes survived in all kinds of harsh environments and spread to oceans far and wide. Their success is so important because these simple organisms are the ancestors of everything that has ever lived on Earth.
The best known prokaryotes are bacteria. As you know, they’re still around just about everywhere. In fact, you’re carrying between 2-6 pounds of the little guys around with you right now! Bacteria are some of the most abundant, successful and adaptive life forms on Earth.
Another lesser known prokaryote is the archaea (pronounced AR-key-ah or ark-EYE-ah). They’re still around, too, and thrive in extreme environments. They flourish near boiling ocean vents, survive in ice floes and live in seas so salty, it would kill anything else. These guys are seriously tough customers. And like bacteria, they live in your gut, too.
But these sturdy, highly adaptable, single-celled organisms were about to face a challenge that nearly wiped them off the face of the Earth.
2. Adapt or die: poison gas nearly wipes out life 2.3 billion years ago
In the first two billion years on Earth, our planet had no free oxygen in the atmosphere. None. Nada. Zip. Most of the oxygen was locked up in the H2O of water molecules and in iron ore. The first organisms were anaerobes, organisms operating in the absence of oxygen, so there was no problem. And life was good.
Then, along came the first prokaryotes to convert sunlight into energy to live. These cyanobacteria, an anaerobe formerly known as blue-green algae, grew like mad. The process of converting the sun’s energy, however, freed up oxygen as a by-product in massive amounts.
The funny thing about oxygen, which is not so funny if you’re an anaerobe, is it’s powerfully corrosive and even toxic. It’s corrosive enough to turn iron into rust, for heaven’s sake. It can mortally damage cells. If you’ve ever wondered why people take anti-oxidants, it’s because free oxygen can cause great harm to living tissue.
And that’s exactly what happened 2.3 billion years ago. Cyanobacteria pumped out so much oxygen that the amount of this poison gas in the ocean went from zero to 1%. Oxygen is so toxic that just 1% in the ocean nearly killed all the prokaryotes on Earth, even the cyanobacteria making all the oxygen!
But a few organisms managed to adapt to this toxic environment, using free oxygen to generate a whole heck of a lot of energy. And those few, strange organisms were about to rock the world.
3. The new cells: the rise of cellular power plants and infrastructure
Imagine a prokaryote cell as a simple tent and campfire. What if it morphed from a tent and a tiny fire into a major metropolis with humongous power plants? Well, that’s exactly what happened about 1.8 billion years ago. Newly-evolved cells, eukaryotes, were immensely more powerful and complex than the humble prokaryote.
Eukaryotes have state-of-the-art protein factories with special folding facilities, delivery services and transportation networks, garbage collectors and recycling centers, and a specialized framework for advanced external locomotion. The command center, the nucleus, doesn’t have a single loop of genetic material but long strands of DNA with the blueprints for the entire operation.
Eukaryotes have so much going on inside, they’re 1000 times larger than their ancestors, the prokaryotes.
And if that isn’t impressive enough, these first, single-celled eukaryotes gave rise to all plants, animals and fungi on Earth.
What makes these complex cells rock? Internal power plants!
Remember all that poison gas that nearly wiped out life on Earth? Eukaryotes use that toxic oxygen to help fuel monster energy generators in the mitochondria.
The mitochondria are believed to be ancient cells from a completely different organism, with its own unique DNA, that were probably gobbled up by an early eukaryote. Instead of getting digested for dinner by the host cell, the mitochondria survived the ordeal and began making energy using oxygen to live. It turned out to be incredibly beneficial for the host to have an internal power plant making scads of energy. All this extra energy fueled the advanced infrastructure inside the cell and helped it move around in the outside environment. So the mitochondria stayed.
When a cell divides to reproduce, the mitochondria does the same, right inside the cell, in every animal and fungus on Earth.
The power generators inside plant cells have a similar story. It’s believed an early eukaryote swallowed up a chloroplast, a tiny cell capable of converting sunlight into energy by photosynthesis.
But if the eukaryote was like a cellular metropolis, the next leap forward would bring about a whole new universe!
4. Living large: from single cells to multi-celled organisms
Take a moment and think about some of your favorite living things. Whether you think of redwoods or puppies, star fish or avocados, they’re all made up of eukaryote cells. The operative word is “cells”- more than one cell- many more than one!
About 600 million years ago, single cells started sticking together with a glue-like protein, making chain-like colonies. It was a safety-in-numbers strategy. These cells were genetically identical, produced by the simple division of a single cell into two clones over and over again. Something amazing happened when these cells not only stuck together but began to communicate with one another and cooperate. The work load and life got so much easier. Muti-celled organisms had evolved!
Some cells lined up with their flagella, their swimming tails, all on one side. Working together, they had more strength and could swim faster. Soon, cells moved beyond long chains and started forming clusters! The cells on the outside could swim and form a protective barrier for the cells on the inside.
The cells on the inside didn’t have to exert any energy to swim but could concentrate on ramping up energy production and waste disposal for themselves and for the outer cells, too.
The advantage of multi-cellularity was immense. One cell no longer had to do everything by itself. It could do just one thing really well. It took the pressure off a poor, single cell. Groups of cells working cooperatively were more efficient. The more efficient the group was, the greater its potential for survival.
The bigger the group of cells became, the harder it was for a single cell to come along and eat it! Even if a part of it got gobbled up, the rest of the cells could survive, as did their genetic information.
Conversely, a large group of cells was no match for just a single cell or a small cluster. Large groups of cells always had more dinner!
The rise of multi-cellular organisms with the specialized cells opened up a world possibilities. For example, what if some cells specialized in reproduction? This one, tiny change in reproductive specialization would bring about possibly the greatest revolution in all of evolution!
5. Genetic diversity gone wild: the dawn of sexual reproduction
Regardless of what you may have heard, the Sexual Revolution actually happened about a billion years ago. But when it did, things got seriously wild, even by prehistoric standards. While this new-fangled strategy for reproducing didn’t have members of the opposite sex at first, because the two sexes hadn’t evolved yet, its impact on genetic diversity was epic.
Sexual reproduction evolved with reproductive cells containing only half of the organism’s usual DNA. That cell would merge with a cell from another organism which also had only half its parent’s DNA. The merger of these two cells created an offspring with a full complement of DNA and was genetically unique from both of its parents. The advantage of this reproductive strategy is huge!
Say an organism has a gene from one parent that’s damaged or has a dangerous mutation. The healthy gene from the other parent might still work fine, and the organism survives to grow and reproduce. Having a set of two different options for every gene greatly increases the chance for an organism’s survival. Only those organisms which live to reproduce pass on their genes.
Taking this a step further, when each generation receives a unique mix of genes, variety within the species flourishes. So when a population is faced with an outside threat, say a new disease or a change in the environment, the chances one of its members has a combination of genes that can survive the danger is greater. This genetic diversity is revolutionary in evolution!
But when you think about it, this method cuts the number of offspring in half. Before the sexual revolution, an organism could just divide itself in half and end up with two cells. Now it takes two cells to create a single new one. That’s 50% fewer progeny. For this to be a successful strategy, the advantage of genetic variation has to be great enough to make up for fewer offspring. And it does!
Genetic variation is such an advantage that today, nearly every living organism on Earth now has its offspring through sexual reproduction.
The consequences of sexual reproduction were an evolution revolution. What happened next was an explosion of kaleidoscopic variety and complexity which would blow life clear out of the water! Life as we know it was just beginning…
Top 10 Evolution Breakthroughs of Life (Part II) begins here.