Fossils show millipede and centipede ancestors evolved legs underwater

In this case, "millions of years" actually means "hundreds of millions of years".

The earliest fossils of centipedes, which were similar to the present house centipedes, are from the late Silurian, more than 400 million years ago. (The long Scolopendra-like centipedes have appeared much later than those similar to house centipedes, presumably as an adaptation to burrowing.)

At the same place lived some relatives of the spiders, named trigonotarbids. Together with scorpions, centipedes and trigonotarbids were the earliest terrestrial predatory animals.

The true spiders, which differ from trigonotarbids mainly by having venom glands and silk glands, appeared almost one hundred million years later, still very early in comparison with most terrestrial animals.

The earliest non-predatory terrestrial animals, which were probably eating mostly rotten vegetation, and which were the prey of centipedes, scorpions and spider relatives, were millipedes, mites, springtails and roundworms.

The parent article is important because for a very long time there were no known aquatic ancestors of the terrestrial millipedes and centipedes, so their origin was a mystery, unlike for the other terrestrial animals. Only in recent years the affinities of some older fossils with millipedes and centipedes have been recognized.