![]() Thanks to their hard skeletons, sponges became the first reef builders on Earth. While they were not played out immediately, there is evidence that parts of instructions for complex bodies were present even in the earliest animals. ![]() Some genes act like orchestra conductors, controlling the expression of many other genes at specific places and times to correctly assemble the components. The assembly instructions for an animal’s body plan are in its genes. The evolution of ever more complex and diverse body plans would eventually lead to distinct groups of animals. The simple body plan of a sponge consists of layers of cells around water-filled cavities, supported by hard skeletal parts. They feed while sitting still by extracting food particles from water that is pumped through their bodies by specialized cells. Although, like other animals, they require oxygen to metabolize, they don’t need much because they are not very active. Oxygen levels in the ocean were still low compared to today, but sponges are able to tolerate conditions of low oxygen. While chemical compounds from sponges are preserved in rocks as old as 700 million years, molecular evidence points to sponges developing even earlier. These clusters of specialized, cooperating cells eventually became the first animals, which DNA evidence suggests evolved around 800 million years ago. Some cells were tasked with making junctions to hold the group together, while other cells made digestive enzymes that could break down food. Living collectively, cells began to support the needs of the group by each cell doing a specific job. ![]() Groups of cells might be able to feed more efficiently or gain protection from simply being bigger. The new complex cells (“eukaryotic cells”) boasted specialized parts playing specialized roles that supported the whole cell.Ĭells also began living together, probably because certain benefits could be obtained. Also, for the first time, DNA became packaged in nuclei. Mitochondria, the organelles that process food into energy, evolved from these mutually beneficial relationships. Something revolutionary happened as microbes began living inside other microbes, functioning as organelles for them. Microbes are just single cells with no organelles and no nuclei to package their DNA. Animal bodies have various cells –skin, blood, bone – which contain organelles, each doing a distinct job. While they can process lots of chemicals, microbes did not have the specialized cells that are needed for complex bodies. However, other innovations were occurring. So, the ocean was still not a suitable environment for most lifeforms that need ample oxygen. In fact, as cyanobacteria died and drifted down through the water, the decomposition of their bodies probably reduced oxygen levels. Rocks dating to after the event do not have iron bands, showing that oxygen was now in the picture.Īfter the initial pulse of oxygen, it stabilized at lower levels where it would remain for a couple billion years more. Rocks dating to before the event are striped with bands of iron. When oxygen is around, iron reacts chemically with it (it gets oxidized) and gets removed from the system. This catalyzed a sudden, dramatic rise in oxygen, making the environment less hospitable for other microbes that could not tolerate oxygen.Įvidence for this Great Oxidation Event is recorded in changes in seafloor rocks. They became Earth’s first photo-synthesizers, making food using water and the Sun’s energy, and releasing oxygen as a result. When cyanobacteria evolved at least 2.4 billion years ago, they set the stage for a remarkable transformation. ![]() Scientists study today’s, rare living stromatolite reefs to better understand Earth’s earliest life forms. Minerals precipitate inside the layers, creating durable structures even as the microbes die off. Stromatolites are created as sticky mats of microbes trap and bind sediments into layers. The signals consisted of a type of carbon molecule that is produced by living things.Įvidence of microbes was also preserved in the hard structures (“stromatolites”) they made, which date to 3.5 billion years ago. The earliest life forms we know of were microscopic organisms (microbes) that left signals of their presence in rocks about 3.7 billion years old. With an environment devoid of oxygen and high in methane, for much of its history Earth would not have been a welcoming place for animals. But that’s not how life on Earth used to be. These and other animals require oxygen to extract energy from their food. Our ecosystems are structured by feeding relationships like killer whales eating seals, which eat squid, which feed on krill. Today we take for granted that we live among diverse communities of animals that feed on each other.
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