Life evolved in the water and later conquered the land. Plants were the first form of life, and without any competition from animals would have been able to spread and diversify. Early plants would have transformed the atmosphere of the planet, making it very rich in Oxygen, at much higher levels than today.
Arthropods were the first animal life on land, and with the Oxygen rich conditions and lack of competition from other animal forms they diversified and some became very large. We know from the fossil record that giant spiders and scorpions were common, and there was a also giant millipede, about 3 meters long. The first amphibians appeared about 390 million years ago. The arthropods and their descendants, insects, were the dominant life forms.
Insects were the first to conquer the sky and about 350 million years ago a truly elegant flying insect Meganeura took to the skies. Meganeura looked very much like a modern Dragonfly and may be related by a common ancestor. Meganeura had a body of about a meter in length and studies of the mandible (jaws) suggest it was a carnivore. Its fossil was originally discovered in Commentry in France in 1880 and a fine specimen was found in Bolsover, Derbyshire, in 1979.