1862, archaeologists discovered the remains of the first archaeopteryx. After recovery, it was found that this prehistoric creature was similar in size and shape to a magpie. It has wide and round long wings and a long tail, but it has sharp teeth on its jawbone, curved claws on its three toes, and its tail is bony, which is completely similar to the tail of a dinosaur. Archaeopteryx fossils have become the strongest evidence that birds evolved from dinosaurs. Archaeopteryx evolved from theropod dinosaurs. The similarities between theropods and birds include: the sole of the foot has three toes, the chest has a "forked bone", and the bone is full of air. In 20 14, researchers from Australia, Britain and Italy discovered the mystery of the successful evolution of theropod dinosaurs-"slimming" movement according to the anatomical characteristics of 1500 of 20 kinds of dinosaurs. It turns out that theropod dinosaur is the only dinosaur whose size is shrinking while other dinosaurs are still growing. Their contraction speed is 160 times that of other dinosaurs. Its average weight was163kg 2100000 years ago, and it has dropped to 0.8kg when it evolved into Archaeopteryx about1600kg years ago. For dinosaurs, it is very important to be very small, which may be the most critical step in the evolution of birds, because small size may be the premise of flight. Although it is huge and can glide, the real flight with wings needs to generate thrust by flapping wings, which requires maintaining a special ratio between the size and weight of wings. Therefore, before the big dinosaur became a bird, it went through a long process of "slimming". Researchers estimate that this "slimming exercise" took about 50 million years for the theropod dinosaur family.