Curious Kids is a series for children of all ages. If you have a question you’d like an expert to answer, send it to [email protected] there already viruses on Earth when the first living cells appeared billions of years ago? – Aayush A., age 16, IndiaHow life on Earth started has puzzled scientists for a long time. And it still does.Fossils provide very important evidence about the evolution of plants and animals. Unfortunately, there are very few fossils of ancient microbes available, so scientists rely on modern microbes to devise theories about how life started. I studied bacteria and another type of microbe called archaea from hot environments for many years to learn how they might have evolved on early Earth, but I still have so many unanswered questions.Based on the fossil evidence we have, single-celled microbes appeared on Earth before larger cellular life like plants and animals. But which kinds of microbes were the very first kind of life?[embedded content]Which microbes are considered alive?Microbes are living, single-celled creatures surrounded by a membrane. They consume and convert nutrients into biological molecules or energy and are too small to be seen without a microscope.By this definition, bacteria, archaea and single-celled eukaryotes are microbes. Bacteria and archaea are single-celled creatures that lack internal membrane-enclosed structures, like a nucleus to hold their genetic material. Single-celled eukaryotes have a nucleus and may have other membrane-enclosed structures.Some scientists consider viruses to be microbes made of genetic material enclosed in a protein coat. They are unable to replicate on their own and hijack the machinery of other cells to make copies of themselves. Because they don’t have many features of living cells, they are not technically alive.Evidence for early life on EarthFossils can provide scientists with clues as to when life started, but they best record hard things like bones and teeth. Microbes are made of soft materials that do not fossilize well. However, some live together in very large groups of cells that can accumulate minerals and leave behind quite large fossils.For example, cyanobacteria formed large structures called stromatolites in the oceans of early Earth. Scientists have found fossil stromatolites that date back to 3.48 billion years ago.Other scientists found what they believe are fossilized archaea in rocks from a 3.4 billion-year-old hot seafloor. The Earth became habitable about 4 billion years ago, so bacteria and archaea must have appeared between 3.5 billion and 4 billion years ago.Looking at the chemical reactions that cells carry out can also provide clues. The reactions that make biological molecules and generate energy make up what’s called the cell’s metabolism. Scientists have found evidence that some metabolic reactions were occurring at least 4.1 billion years ago. These reactions may have been occurring on their own before cells had evolved, perhaps on the surfaces of clays or minerals.Theories about how life started on EarthCells copy their genetic material, made of DNA and RNA, to pass it on to new generations. Although DNA is the form of genetic material most living organisms use today, some scientists believe th …
