The idea of birds as a kind of dinosaur was revived in 1969 by paleontologist John Ostrom. He brought out the idea that dinosaurs may have been warm-blooded, thus, more active and similar to birds. Robert Bakker’s article in Scientific American the next year pursued the same ideas. At this time, scientists also began to delve deeply into dinosaur physiology (cells and tissues), noticing the physiological similarities and differences between the animals and other species such as birds. It is these studies, in combination with skeletal evidence, that scientists hope will lead to the correct answer concerning bird lineage.