What was one of the most important steps in the evolution of early
living things?

the development of the enzyme Rubisco and evolution of oxygenic photosynthesis.

Explanation: One can pick many key steps in early evolution: the development of the first cells, oxygenic photosynthesis, endosymbiosis leading to eukaryote organisms.

One important step was the mutation to the enzyme ribulose bisphospate carboxylase oxygenase (Rubisco) which enabled oxygenic photosynthesis.

The cyanobacteria that had Rubisco changed the planet’s atmosphere by producing oxygen as a by-product of photosynthesis. They were the most successful organisms of the time and have continued as a successful and ubiquitous group of organisms for billions of years.

The production of oxygen changed the direction of evolution profoundly. Later endosymbiosis led to the evolution of plants, with cyanobacteria the ancestors of chloroplasts, making Rubisco one of the most common enzymes on the planet.