Animal Life On Earth
On the Planet Earth, approximately 7.77 million animal species are present. The range of the body of animals is 0.00033 inches to 110 feet. Animals are multicellular eukaryotes( any cell or organism that possesses a clearly defined nucleus.) whose cells are bound to each other by collagen.
The Beginning of Animal Life
The environment was without oxygen and high in methane, and the history of Earth tells us it would not have been a welcoming place for animals. The earliest life forms were microscopic organisms 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. Approximate 2.4 billion years ago cyanobacteria evolved and became the Earth’s first photo-synthesizers, making food using water and the Sun’s energy, and releasing oxygen as a result.
Multicellular Life of Animal
Animal bodies have different cells, skin, blood, and bone which contain organelles, each doing a different job. Microbes are just single cells with no organelles and no nuclei to package their DNA. And a revolution occurred when 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, and new complex cells were made.
The First Animal on Earth
Cells also began living together and making clusters and these clusters organized, cooperating cells eventually became the first animals, whose DNA 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. In the fossil record, some early animals have been preserved, but the vast majority have not.