I had previously written about creation of our Earth, and now how early life was formed. However, with an environment barren of oxygen and high in methane, for much of its history, Earth would not have been a welcoming place for animals. The earliest life forms we know of were microscopic organisms or microbes that left signs of their presence in rocks about 3.7 billion years old. They were a type of carbon molecule that is produced by living things.
Evidence of microbes was also preserved in the hard structures called stromatolites which date to 3.5 billion years ago. Stromatolites were formed as sticky mats of microbes trap and bind sediments into layers. Minerals ‘precipitate’ inside the layers, creating durable structures even as the microbes die. Scientists study today’s, rare living stromatolite reefs to understand the Earth’s earliest life forms.
When cyano-bacteria was formed at least 2.4 billion years ago, they set the stage for a remarkable transformation. They became Earth’s first photo-synthesizers, making food using water and the Sun’s energy, and releasing oxygen as a result. This catalyzed a sudden, dramatic rise in oxygen, making the environment less hospitable for other microbes that could not tolerate oxygen.
Evidence for this ‘Great Oxidation Event’ is recorded in changes in sea-floor rocks. When oxygen is around, iron reacts chemically with it and it gets oxidized and gets removed from the system. Rocks dating to before the Oxidation Event are striped with bands of iron. Rocks dating to after the event do not have iron bands, showing that oxygen was now in the picture. After the initial pulse of oxygen, it stabilized at lower levels where it would remain for a couple billion years more. In fact, as cyano-bacteria died and drifted down through the water, the decomposition of their bodies probably reduced oxygen levels. So, the ocean was still not a suitable environment for most lifeforms that need ample oxygen.
However, other innovations were occurring. While they can process lots of chemicals, microbes did not have the specialized cells that are needed for complex bodies. Animal bodies have various cells like skin, blood, bone which contain organelles, each doing a distinct job. Microbes are just single cells with no organelles and no nuclei to package their DNA. Something revolutionary happened as microbes began living inside other microbes, functioning as organelles for them. Mitochondria, is the organelles that process food into energy, evolved from these mutually beneficial relationships and for the first time, DNA became packaged in nuclei. The new complex cells called “eukaryotic cells” boasted specialized parts playing specialized roles that supported the whole cell.
Cells also began living together, probably because of certain benefits that could be obtained. Groups of cells that might be able to feed more efficiently or gain protection from simply being bigger. Living collectively, cells began to support the needs of the group by each cell doing a specific job. Some cells were tasked by making junctions to hold the group together, while other cells made digestive enzymes that could break down food.
These clusters of specialized, co-operating cells eventually became the first animals, that DNA evidence suggests evolved around 800 million years ago. Sponges were among the earliest animals, while chemical compounds from sponges are preserved in rocks as old as 700 million years, molecular evidence points to sponges developing even earlier. Oxygen levels in the ocean were still low compared to today, but sponges are able to tolerate conditions of low oxygen. Although, like other animals, they require oxygen to metabolize, they don’t need much because they are not very active. They feed while sitting still by extracting food particles from water that is pumped through their bodies by specialized cells. The simple body plan of a sponge consists of layers of cells around water-filled cavities, supported by hard skeletal parts. The evolution of ever more complex and diverse body plans would eventually lead to distinct groups of animals.
The ‘assembly instructions’ for an animal’s body plan are in its genes. Some genes act like orchestra conductors, controlling the expression of many other genes at specific places and times to correctly assemble the components. While they were not played out immediately, there is evidence that parts of instructions for complex bodies were present even in the earliest animals. The Cambrian Period (541-485 million years ago) saw a wild explosion of new life forms. Along, with new burrowing lifestyles came hard body parts like shells and spines. Hard body parts allowed animals to more drastically engineer their environments, such as digging burrows. A shift also occurred towards more active animals, with defined heads and tails for directional movement to chase prey. Active feeding by well-armored animals like trilobites may have further disrupted the sea floor that the soft Ediacaran creatures had lived on.
However, despite all the changes that were to come, by the end of the Cambrian Period, nearly all existing animal types, or phyla, like mollusks, arthropods, annelids, etc., were established, and food webs were emerging, forming the foundation for the ecosystems on Earth today.