The History of the Earth at an Arms Length
30 Oct 2019|Brooke Johnson
I study the origin of complex life. I want to understand why it took so long to go from simple single cell organisms, such as amoebas and algae, to things like multicellular organisms like plants and animals. I do this by examining 1350-million-year-old mud from northern Australia by a variety of means. This ranges from basic observations of colour and texture, through to advanced analytical techniques requiring esoteric machines to measure isotope ratios or minor variations in the abundances of different minerals.
Working with deep time can skew your perceptions without you realising. I now think of anything that takes less than 20 million years as being “really quick”. If you’re not familiar with the term ‘deep time’, then it basically refers to everything older than 2.5 million years ago. Things younger than that we refer to as geography, or archaeology, or history. For some one like me, that works at the deep end of deep time, even the ubiquitous dinosaurs, the pop-culture standard for Really Old, are relatively new. The period of time I study, is around five times older than the oldest dinosaur, and twice as old as the first creatures you could even charitably refer to as animals. The world I study has been gone for over a billion years, it is a world utterly alien to our own. It is both further than the most distant star, and closer than the thickness of a shadow, to paraphrase the late Terry Pratchett.
My brain is fully acclimatised to such chasms of time, but the average person e.g. my dad, is not. So, here is a wee quick visual exercise that might help you truly grasp the age of the Earth, and our place within it. Feel free to join in at home.
Hold your arm out with your fingers stretched, which ever you like, your choice. Place the tip of the index finger of your other hand in the middle of your chin, this the formation of the Earth, ~4560 million years ago. The next check point is where your neck and should meet. Here the Earth’s surface first solidifies and is then destroyed when an object the size of Mars collides with the Earth. The debris from this collision becomes our familiar moon. Next, we come to your armpit, it is here, around 4000 million years ago, that the first life appears.
About 3500 million years ago, round about your upper bicep, oxygen is suddenly more abundant in the Earths atmosphere and the chemistry of our planet is changed forever. Currently the blame for this ‘Great Oxidation Event’ is placed on the evolution of photosynthetic cyanobacteria. They have yet to apologise. After this, nothing much happens until we get to your elbow, around 1600 million years ago. A bacteria swallows another bacteria, and for some reason the larger bacteria does not eat its victim. This is endosymbiosis, the evolution of the complex eukaryotic cell, the smaller bacteria eventually becomes the mitochondria of our own cells. You would think an event like this would revolutionise biology, and it does eventually. But like the bacteria before them, the first eukaryotes do nothing much for a really long time.
This period of ‘nothing much for a really long time’ lasts from your elbow to your wrist, around a billion years. It is this interval of time, known as ‘The Big Lazy’ by me, and the Mid-Proterozoic by everyone else, that is the focus of my research. By studying rocks of this age, I am trying to understand if environmental controls, like nutrient availability or temperature (it was really warm back then) could have caused the apparent stasis in eukaryote evolution.
From around 800 to 600 million years ago, the base to the top of your wrist, a lot happens. Some eukaryotes do a second endosymbiosis with cyanobacteria and become photosynthetic algae, some also start working together in a single body and become the first sea weeds, who also invent sexual reproduction. Some amoeba start eating other amoeba, who in response either make the first shells, or learn to swim. This triggers a predator-prey arms race that rapidly escalates into a savage war for survival that rages to this day. At 550 million years, the base of your palm, some of the combatants grow large and start leaving visible fossils in the rocks. Some argue these creatures are plants or fungus, but most agree they are simple animals, or at least animal style organisms. At 542 million years, the middle of your thumb pad, life enters its nuclear age. Large shelled organisms appear, things with eyes and mouths and teeth and perhaps the greatest biological innovation of them all; an anus that is separate from the mouth.
By the top of your palm, large life forms are everywhere. They fill the oceans, they cover the land, they have infiltrated every single niche available, and they have made as many new niches. In the middle segment of your index finger, big lizards appear. A bit later they vanish when a rock the size of small mountain, and travelling at 250 thousand km per hour, tries to visit Mexico. Very quickly the lizards are replaced by furry warm-blooded, live birthing organisms called mammals. Around the middle of the last segment of your index finger the Earths vast tropical forests are replaced by grass lands. As grass is shorter and harder to climb than trees, some of the arboreal mammals called apes decide to live on the ground. To avoid being eaten they start walking upright to see over the grass. By the last quarter of the last segment of your index finger, some of those apes have started using simple stone tools to help them catch and prepare food. Shortly after, they also figure out how to make fire and how to attach a sharp stone to a long stick so they can catch food from a distance. Suddenly the Earth gets very cold and the upright apes decide to leave their warm homeland and travel to some of the coldest parts of the Earth. I have personally never forgiven them.
This brings us to the very end of your index finger, the final fraction of a millimetre of your fingernail. This thinnest sliver of your body, and of Earth History, represents all of recorded human history and civilisation, it represents every ‘modern’ human who has ever lived or ever will live. Around the last 300 thousand years or so. The entirety of human history, both written, archaeological and biological, is just the thinnest wisp of breath in the lifetime of our planet. A planet, which is itself, just a single rocky mote in the vast sweep of the universe.
Brooke Johnson is a geologist researching the co-evolution of early eukaryotic life and Earth surface environments. His work combines classical field geology and petrographic observation, with high resolution geochemical analysis in order to understand the role of the environment in shaping the evolution of early complex life.