Since the mid to late twentieth century, evolutionary theory has undergone a number of challenges due to advances in biology, biochemistry, and genetics. But its biggest hurdle stems from the question, “How did life get here in the first place?”
Prior to Darwin’s theory, scientists thought life could spontaneously generate from non-life - meaning for example that dust could literally come to life as fleas. However, just a few years after the publication of the theory of evolution, French biologist, microbiologist, and chemist Louis Pasteur, conducted a number of experiments that cast doubt on spontaneous generation. His discoveries led to the Law of Biogenesis, which states that life only comes from life.