Final answer:
The first genetic material in early organisms on Earth is believed to have been RNA, supporting the 'RNA world' hypothesis. The idea posits that RNA could both hold genetic instructions and catalyze biological reactions prior to the evolution of DNA and proteins. The oldest known life forms, prokaryotic cells, appeared about 3.5 billion years ago.
Step-by-step explanation:
It is now thought that the first genetic material in the earliest living organisms on Earth was likely RNA (ribonucleic acid). Unlike DNA, RNA is a molecule that can perform dual functions: it can hold genetic information and carry out cellular activities, such as catalyzing chemical reactions.
The widely accepted 'RNA world' hypothesis suggests RNA-based life might have existed before DNA and proteins became the main actors. This is because RNA can store genetic information similarly to DNA and can also catalyze chemical reactions like proteins, albeit less efficiently. The idea is supported by the discovery of ribozymes, which are RNA molecules with enzymatic properties. This hypothesis fits well with modern genetic findings that show RNA as an intermediate between DNA and proteins in cellular processes.
The path from inorganic molecules to the first life forms remains a subject of intense research and speculation. Scientists like Stanley Miller and Harold Urey demonstrated that organic compounds necessary for life could form under early Earth conditions. Life began to emerge around 3.5 billion years ago with prokaryotic cells, later followed by the more complex eukaryotic cells. During this period, the Earth's atmosphere and environment were constantly changing, with early life forms adapting accordingly, setting the stage for the rich diversity of life we see today.