Muscles first evolved over 600 million years ago, likely in simple multicellular animals before the rise of true bilaterians.
• Early Contractile Proteins (~1 billion years ago)
The molecular machinery for muscle contraction, such as actin and myosin, existed in single-celled eukaryotes before multicellular animals evolved. These proteins helped cells move and change shape.
• First Multicellular Muscle Precursors (~700 million years ago)
Some of the earliest animals, like sponges, lack muscles, but their cells still use actin and myosin for limited movement. However, cnidarians (jellyfish, corals, and anemones) developed primitive muscle-like cells, called myoepithelial cells, for movement.
• True Muscles in Bilaterians (~600–550 million years ago)
The first true muscle tissues evolved in early bilaterians, animals with a symmetrical body plan, such as flatworms. These muscles were derived from the mesoderm, a key embryonic layer that gave rise to more advanced muscle structures.
• Advanced Musculature in Cambrian Animals (~520 million years ago)
During the Cambrian Explosion, more complex animals like arthropods and early vertebrates evolved specialized muscle groups, enabling stronger movement, burrowing, and swimming.
Muscles have since diversified into smooth, cardiac, and skeletal types in vertebrates, allowing for increasingly sophisticated movement and bodily functions.