History of Animals

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Our solar system, our sun and planets formed about 4.6 billion years ago, it was a long process, which took about 100 million years to form. Almost nothing is known in the first 600 to 800 million years of Earth’s history except we know the surface was engulfed in blazing heat from the inner processes of earth development, covered in deadly radiation from the sun’s ray, and clashing with enormous meteorites. Around 3.9 billion years ago, Earth was cooled enough to form a thin crust on the surface.

Once that happened, there were life appearing in different part of the planet.

First life

During that time the atmosphere contains carbon, many different minerals and there were plenty of water. At the time of first life, the temperature was extreme, the surface of Earth was very hot that water evaporated quickly or it was so cold that it froze. This type of temperature was essential for the beginning of life on Earth.

All life first begins in the oceans, because water protects life from the sun’s radiation but at the same time able to use the sun’s radiant as energy. The earliest fossils of a living organism go back to 3.5 billion years. These first living organisms are like today’s cyanobacteria, but back then, they were prokaryotic, photosynthetic cells. (Prokaryotes are the most primitive cells, lacking the specialized units called organelles found in later, more complex cells.)

Anything dating back beyond 3.5 billion years was never found, however these fossil cells already developed shells that leads scientists to believe there were other living organisms living on earth much earlier, perhaps 3.8 billion years ago, right after the Earth’s crust was form. Scientists’ hypothesis that the first cells were heterotrophs feeding on organic matter, and later photosynthesis came about.

Age of bacteria

During the early stages of first life on earth, the most dominate life were the cyanobacteria. For about 2.5 billion years, it was releasing oxygen and building the supply of that gas in the atmosphere. Oxygen breathing organisms first appeared 1.5 billion years ago, when eukaryotic cells (cells with a nucleus and other organelles) first appeared.

Around 1.2 billion years ago, a number of eukaryotes are thought to have taken in smaller photosynthetic cells or cell units that went living inside some eukaryotes in a symbiotic union that benefited both. Consequently, it was the start of a strain that ended up becoming plants. A few of the eukaryotic cells that remained heterotrophs became the ancestors of animals.

Multicellularity

The presumably method for the rise of multi-cellular animals is because of the combination of the previous independent and free-living one-celled organisms in the form of a colony. As members of a colony, the cells began to specialize and eventually lose the ability to live on their own.

The simplest of all animals are sponges, let’s use it as a living model. For example, if a piece of a sponge was torn away, the cells within the sponge will start to move randomly. When the cells encounter another cell, they stop moving and form clusters of cells. When there are enough cells formed to form an adequate mass, each cell seems to remember its unique arrangement and function. Then the cells push on until they are back to their correct places so the sponge is back functioning.

One-cell organisms benefit from different advantages. Microorganisms could survive a nuclear holocaust that destroyed the multi-cellular life. Furthermore, one-cell organisms may be considered immortal. When a protozoan divides, the daughter cells are equally as old or as young.

Microorganisms do die from dehydration, predation, and in many other extreme environment, except it does not have a natural cause of death.

So why some cells mass together to form multicellularity?

The main benefit of multicellular organisms is the size. Multicellular organisms are bigger than one-cell, that mean they are likely to eat something rather than being eaten. Also, multicellular organisms have better control over their surroundings than microorganisms because of their superior mobility, which allow animals move away from harmful environments or to flee from predators or to protect themselves.

However, there are some drawbacks with multicellularity. One drawback is that multicellularity is expected to die. All multicellular animals have a limited life span. They are born, grow, age and die.