What real evidence do scientists have that living things have changed over time?

The first living things that we know of were prokaryotes (single celled organisms without a cell nucleus), and they appeared about 3.8 billion years ago. About 3 billion years ago, the first prokaryotes evolved the ability to make food using sunlight, air, and water (photosynthesis). About 2 billion years ago, the first eukaryotes (organisms with a cell nucleus) evolved, and the first multicellular organisms evolved 1 billion years later. Approximately 500 million years ago, the Cambrian explosion occurred, and within only 140 million years of that (360 million years ago), plants, amphibians, insects, and fish lived on Earth, along with most other major invertebrate groups. Amphibians evolved into reptiles about 300 million years ago (mya), and reptiles diversified into mammals (about 200 mya), dinosaurs (225+ mya), crocodillians, and a variety of other groups that are now extinct. Dinosaurs evolved into birds about 150 mya.

Meanwhile, the simple, seedless plants from 300 mya evolved into conifers , cycads (less than 300 mya) and ginkgos (199 mya), and then flowering plants appeared about 130 million years ago, one group of which was the Magnolids.

After the K-T extinction 65.5 mya that wiped out the non-avian dinosaurs, the tiny mammals quickly evolved into larger and more diverse forms, and finally into the mammals that are alive today.