Origins

The Earth as we know it contains a stunning array of life. From bubbling geothermal vents deep in the ocean to tropical rain forests, life has evolved to cover every nook and cranny on Earth’s surface. It’s hard to imagine our planet without this diversity of life; as a surface only covered with bare rock and water, but for a very long time that is all Earth was.

Thanks to Charles Darwin (ecogeek) and Alfred Russel Wallace (the forgotten ecogeek), we know that evolution by natural selection was the mechanism by which life was able to become so diverse. Exactly how and when life originally formed, however, is still being debated and will likely be debated for a long time. First of all, nobody was there to witness it, and the information that was recorded in the rocks is difficult for scientists to decipher. But we do know that it happened!

In the beginning…

Earth 4.6 billion years ago was not a pleasant place to be. It had just been formed, and it was still mostly molten rock that had not yet separated into the core, mantle and crust as we know it. It was constantly being hit with asteroids and huge volcanoes and fissures on the surface were belching lava, water vapor and toxic gasses. This is called the Hadean Eon from the Greek word Hades which means ‘underworld’.

Over the next 600 million years, though, the hot molten rocks began to separate and form the iron core, mantle and a crust that finally began to cool and harden. The volcanoes that had been erupting for hundreds of millions years had put a lot of water vapor into the atmosphere. When the Earth was finally cool enough, that water vapor began to condense and rain down on Earth, forming the oceans. At this time in Earth’s history, there wasn’t much dry land and oceans covered almost everything. The atmosphere was mostly carbon dioxide, nitrogen and water. Conditions were ripe for some life-making.

Creation

The first thing you have to remember about the origins of life is that it was not some quick magic trick that the Earth pulled off. It was a very gradual process of shuffling and recombining basic molecules until they became more and more complex. These complex molecules then needed to replicate themselves “on purpose” and have a built in genetic code that was able to construct bigger molecules from scratch. Then, it needed to encase itself somehow in a semi-permeable membrane. The fact that this happened at all is a miracle, but it did!

The most probable explanation for this, without getting into specific chemistry, is: The basic molecules that existed on early Earth (nitrogen, water, carbon dioxide, methane) were able to use energy from deep sea vents, lightning, or the ultraviolet radiation from the sun to create more complex organic molecules like sugars, amino acids and fatty acids. Laboratory experiments performed by A.I. Oparin and Stanley Miller have shown that given the right circumstances it is possible to form complex organic molecules from basic inorganic ingredients.

Once these molecules were formed they then needed to become concentrated enough to form extra large molecules like nucleic acids and sugars, the building blocks of RNA and DNA. Certain areas, like deep sea vents or areas with high concentrations of clay minerals have been known to naturally concentrate these organic molecules. RNA was most likely formed before DNA, because it is able to replicate itself without too much help (DNA needs a lot of help to replicate… mostly from RNA). If RNA was formed first, it would then begin to replicate itself and begin making proteins from amino acids. Over time, the RNA strands along with the proteins they produced would be able to make a double stranded DNA molecule capable of replication.

If a DNA strand was then encased in a fatty membrane we could technically call this a cell. Laboratory experiments have shown that a fatty membrane with small amounts of DNA and proteins is subject to osmosis. Water would rush in to balance the concentration of solute inside and outside the cell and as DNA kept replicating, more water would rush in and the fatty membrane would eventually split. Primitive cell replication!

The Familiar

With all the requirements of life being filled, early cells would be subject to natural selection, eventually producing bacteria. The oldest types of bacteria that are still around today are called Archaebacteria, followed by Eubacteria. Together, these two categories make up the two domains of living things. Every living thing on Earth can trace their lineage all the way back to these two basic types of organisms!

Improbable? Yes. Impossible? No.

As we have said, the fact that any of this even happened is a miracle. Or is it? Think of it this way, the Earth was formed approximately 4.6 billion years ago. Evidence for very primitive forms of life like archaebacteria arose about 4 billion years ago. That leaves 600 million years for all this to take place. In that amount of time even very improbable things can happen, especially if each successful step increases the probability of success for the next step.

Lets say you buy a lottery ticket with a one and a hundred million chance of winning. Stupid move right? Now lets say every time you win even a little bit of money, your chances of winning the next time you buy a ticket go up. Now go buy a lottery ticket every day for 600 million years. Do you think you will win the Jackpot?

“There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved. “- Charles Darwin

Video:

Life can get tricky sometimes. I mean the history of life, of course… The Cambrian Explosion is a great example. Evidence for the first bacterial forms living on Earth date back to 4 billion years ago. It then took nearly 2 billion years for these unicellular organisms to get it together, as they say, and become multicellular. Then, after only a billion and a half more years (that’s 500 million years ago for those playing at home) nearly every animal phylum that exists today had evolved and diversified to some extent. This was quick jump for a slow and gradual process.

The sudden appearance of diverse life in the fossil record baffled scientists and even caused Darwin to question his theory on evolution by natural selection. They called this the Cambrian Explosion. Today, scientists have several theories to explain the phenomenon. One theory is that life may have been able to evolve quickly because the Earth had changed so much that it created enough new niches that species could rapidly fill them. The second theory is that the explosion was really a dud; that some lineages can be traced farther back than the Cambrian and so there was no real ‘explosion’. Like a lot controversial science topics, the answer probably lies somewhere in the middle.

In the last 542 million years there have been three eras of Earth’s history, the Paleozoic, the Mesozoic, and the Cenozoic. Each of these eras is broken up into two or more periods. The Cambrian period is the first period of the Paleozoic era, the period in Earth’s history when we can see more evidence for diverse life than anytime before that. This period began 542 million years ago with an explosion of life and ended with a mass extinction 52 million years later.

Building a Bomb

Earth was changing drastically in the millions of years before the Cambrian Explosion. Multicellular animals had taken some strange forms, and many of them have not been fully classified by scientists because they are too weird to fit any one category. Some animals, like flatworms, segmented worms and maybe cnidarians and sponges (though this is up for debate) made their appearance just before the Cambrian. These animal-wierdos are known as the Ediacaran Fauna. Unfortunately, these animals were nearly all soft bodied and didn’t fossilize well. This is one of the arguments against the explosion theory; that the fossil record is too incomplete to say that animals diversified as fast as it appears. It is possible that we evolved from stranger beings than we thought.

Life on Earth today would probably have been much different if the Ediacaran Fauna had persisted and lived on. But, fortunately for us, they did not. Geologists agree that, before the Cambrian, in the late Proterozoic era, Earth’s temperature dropped insanely low and glaciers covered almost the entire surface of the planet. Temperatures got so low that even the equatorial areas were a slushy mess. This is known as the Snowball Earth theory. The massive glaciers that covered Earth may explain why Earth gave the cold shoulder (get it?!) to the Ediacaran Fauna and many went extinct. Thankfully, the ice thawed and life was able to move on.

The late Proterozoic was changing shape as well as temperatures. A massive supercontinent called Rodinia began to fracture and split into separate continents. This allowed shallow seas to dominate a significant chunk of Earth’s surface. Today, shallow seas are biodiversity hotspots for a reason. They are warm, they have plenty of sunlight, and plenty of food. Surviving Ediacaran fauna would have naturally migrated to such a place. Because many ecological niches were empty after the last major extinction, these animals would have been able to radiate easily and speedily.

Who came to the big bang?

As we have said, some phyla like flatworms, segmented worms, sponges and cnidarians, hung around from the Proterozoic. Others, like mollusks, arthropods and chordates evolved in the Cambrian.

Mollusks are represented by what scientists call ‘small shelly fauna’. Basically, they are just like they sound: small (a couple of millimeters long), soft bodied organisms encased in a calcium carbonate shell. Later in the Cambrian, more shelled fossils appear.

Arthropods are represented by everybody’s favorite fossil, Trilobites, among other animals that evolved a hard chitin exoskeleton, instead of a calcium carbonate shell.

The earliest Chordates, our own illustrious phylum, even made their appearance in the Cambrian! This animal was called Pikaia, and was one of the earliest true chordates.

The list goes on to include nearly every animal phyla that currently exists! The only phylum that was tardy to the party were the Bryozoans (moss animals), who evolved in the early Ordovician period, right after the Cambrian.

Fat Man and Little Boy

Whether the phyla that appear are a result of long, gradual evolution or fast radiation, we can see that the fossil record becomes much more crowded in the Cambrian. Much of this is due to animals finally evolving hard parts like exoskeletons and shells, which fossilize very well.

Many scientists call the Cambrian Explosion an ‘evolutionary arms-race’ to account for such a fast radiation. The adaptations that arose in the Cambrian were mainly for protection and predation. If one animal evolved a hard exoskeleton, another would have to evolve better predation strategies to survive. Some evolutionary marvels like eyes, brains and ears may have come out of such arms races.

A Form of Natural Selection

Have you ever wondered why a peacock’s feathers are so big and gaudy? Or why a Lyrebird’s song is so complex? Maybe you’ve simply been outraged by sexual dimorphism in general. Well the answer to your questions and outrage is Sexual Selection, a more specific form of Natural Selection.

Natural Selection is a process in which favorable traits are maintained in a population, and subsequently passed down through generations, while unfavorable traits are eliminated from a population over time. Organisms that exhibit traits that help them to survive, live longer and are able to pass those good genes to the next generation. Organisms that exhibit traits that are to their detriment generally do not live as long and do not pass on their negative genes.

A Competitive Nature

All organisms are in a never-ending competition for survival and reproduction. Often times, the competition is based on exploiting food resources in a given habitat. For example, a squirrel’s memory helps him to remember where he hid his nut, and a deer’s strong, flat teeth allow her to munch continuously on vegetation. The squirrel is in competition with other squirrels for access to nuts, and the deer is in competition with other deer for access to veggies. This is called intraspecific competition, where one species competes with other members of their own species for resources. If the squirrel and the deer directly competed with one another, it would be called interspecific competition.

Sexual Selection

The competition for limited resources is also called “The Survival of the Fittest”. “The Fittest” do not always have to be those organisms that could gather the most resources, though. Remember, characteristics are preserved in a population by passing them down through the generations. If an organism is unable to reproduce, it doesn’t matter how well they can gather food resources, their traits are not going to stay in the gene pool for long.

Sometimes, favorable traits are those characteristics that help the organism attract attention from the opposite sex, thus ensuring their genes stay in the pool. Sexual selection is a form of intraspecific competition that favors traits for reproduction and raising offspring. The most drastic examples occur in polygamous species, those species that have more than one mate in their life. Characteristics that have been sexually selected for in a population generally come in two different flavors: Weapons and Ornaments.

Weapons and Ornaments

Weapons are traits that individual males possess to help them defend their territory against rival males. For example, male deer have large antlers on top of their head to both mark their territory and to fight rival males. Those individuals that have the largest rack will generally keep their territory and have their choice of female mates. If he can keep his territory defended, the genes that code for large antlers will be preserved and his offspring will hopefully be as reproductively successful.

Weapons can sometimes get a little out of hand. The Irish Elk is example of an animal that was sexually selected into extinction. Their antlers were size-selected for so long that the rack got too big for the body and they were unable to support themselves. Many paleontologists believe this contributed to their extinction. How like a man… We just keep pushing it don’t we?

Ornaments are features on an organism’s body that stand out (Juicy Couture) and help potential mates decide if the suitor is worthy of mating with. Ornaments can be a number of different decorations or behaviors. Bright colors, like those on the face of a male Mandrill, can often give insight into how healthy that particular male it. Birds of Paradise often have long colorful feathers, like Peacocks, to impress and attract females.

Some behaviors can be thought of as “ornamental”, like the song of a lyrebird, the agility of a swallow, or the beautiful dancing grebe. Mammals may not have the most obviously sexually selected traits, but remember that vision is not your only sense. Mammals are very sensitive to smells and many of the traits that we select for are based on the release of pheremones. Think about that the next time your attention is diverted towards someone with a sweet smelling perfume, or cologne, they may just be trying to cheat selection.