This is the highest mountain on Earth.
Many living things exist on Earth.
This is the tallest mountain on Earth.
All living things on Earth need water.
地球 is the noun for 'the Earth' (the planet itself), while 地球上 is the locative expression meaning 'on Earth/on the earth's surface'; learners sometimes drop or add 上 incorrectly.
Compound of 地球 (chikyū, 'Earth') + 上 (jō, 'on, above'). The reading じょう is the standard on'yomi for 上 in this context.