Approximately when did the first plant and arthropods appear on land

470 million years ago

Explanation:

Both the plants and the arthropods initially developed and lived in water. As the atmosphere was changing though, they gradually started to develop for life on land, which eventually gave birth tot he first plants and arthropods that were fully terrestrial. The first plants on land occur around 470 million years ago in the fossil records, while the first arthropods around 450 million years ago, though the genetic analyses have shown that they were present on land around 20 million years earlier. The first terrestrial plants were small, simple, and had small and not well developed roots. The first arthropods though, the millipedes, centipedes, and scorpions, were almost identical to the ones we have in the present, being well developed and adapted for terrestrial life.

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