Scientists infer that oxygen first began to enter Earth's atmosphere after the appearance of

Oxygen entered the Earth's atmosphere after the appearance of the Great Oxidation Event (Great Oxygenation Event). The Great Oxidation is the first mass extinction on Earth.

The Great Oxidation lasted 2.4 to 2.0 billion years ago.

Oxygen appeared on Earth thanks to cyanobacteria using photosynthesis.

advantage of using stem cells in medical research:

1.       it provides medical benefits in the fields of therapeutic cloning and regenerative medicine.

2. it provides great potential for discovering treatments and cures to a variety of diseases including parkinson disease, spinal cord injuries, diabetes, alzheimer’s disease and many more.

3. limbs and organ could be grown in lab from stem cells and then used in transplant or treat illness.

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