Explain why sponges and cnidarians were the first animals to evolve

Once the Earth was full of water (ocean) after volcanic activities the mainland became land (sorry about my english i am hungarian and i am still learning the language)

Sponges and cnidarians were the first animals to evolve from a multicellular ancestor. The oldest fossil evidence of multi-cellular animals is burrows that appear to have been made by smooth, worm like organisms. Between 620 and 550 million years ago large, complex, soft-bodied multi-cellular animals appear in the fossil record.