The universe has existed for about 13,500,000,000 years. The Sun and the Earth have existed for about 4,500,000,000 years. Unicellular organisms on Earth have existed for about 4,000,000,000 years. Multicellular organisms on Earth have existed for about 3,000,000,000 years. Complex life on land has existed for about 500,000,000 years. Mammals without dinosaurs (excluding birds) have […]