Final answer:
The least diverse era for DNA of life forms was the Hadean Eon (4 to 4.6 billion years ago), a time when Earth was cooling, frequently impact-sterilized, and predominantly hosting simple microbial life. It was followed by the Archaean Eon marked by the appearance of stromatolites, and later by eras of increased biodiversity due to evolution.
Step-by-step explanation:
When considering the diversity of life on planet Earth, it is important to look back billions of years to the earliest stages of life's existence. The least diverse era for the DNA of life forms was during the very beginning, in the period known as the Hadean Eon, stretching from 4 to 4.6 billion years ago. During this time, the Earth was still cooling and the conditions were harsh, with the surface being impact-sterilized regularly by large heavenly bodies, possibly wiping out nascent life forms and delaying the onset of diversity.
After the Hadean Eon, life began to flourish in the Archaean Eon, approximately 3.5 to 4 billion years ago, as evidenced by the existence of stromatolites built by colonies of photosynthetic cyanobacteria. However, in comparison to later periods in the history of life such as the Paleozoic Era, when life exploded into a vast array of forms and eventually transitioned onto land, the Archaean Eon represented a time with considerably less diversity. Life during the Archaean was predominantly microbial, with primarily single-celled organisms, and lacked the complex multicellular organisms that would later emerge.
Overall, the diversity of life has increased exponentially over geological time scales, with the first multicellular organisms appearing much later, prompting a significant increase in biodiversity. These changes were driven by the evolutionary processes, enabling life to branch out into the myriad forms we are familiar with today.