HOW FIRST ORGANISM IS FORMED??
Answers
Answer:
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. The signals consisted of a type of carbon molecule that is produced by living things.
Explanation:
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Today we take for granted that we live among diverse communities of animals that feed on each other. Our ecosystems are structured by feeding relationships like killer whales eating seals, which eat squid, which feed on krill. These and other animals require oxygen to extract energy from their food. But that’s not how life on Earth used to be.
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. 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. The signals consisted of a type of carbon molecule that is produced by living things.
Evidence of microbes was also preserved in the hard structures (“stromatolites”) they made, which date to 3.5 billion years ago. Stromatolites are created as sticky mats of microbes trap and bind sediments into layers. Minerals precipitate inside the layers, creating durable structures even as the microbes die off. Scientists study today’s, rare living stromatolite reefs to better understand Earth’s earliest life forms.
An Oxygen Atmosphere
When cyanobacteria evolved 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 seafloor rocks. When oxygen is around, iron reacts chemically with it (it gets oxidized) and gets removed from the system. Rocks dating to before the 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 cyanobacteria 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.
Multicellular Life
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 –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, the organelles that process food into energy, evolved from these mutually beneficial relationships. Also, for the first time, DNA became packaged in nuclei. The new complex cells (“eukaryotic cells”) boasted specialized parts playing specialized roles that supported the whole cell.
Cells also began living together, probably because certain benefits could be obtained. Groups of cells 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 with making junctions to hold the group together, while other cells made digestive enzymes that could break down food.
The First Animals
These clusters of specialized, cooperating cells eventually became the first animals, which 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.