What the word names now
A biologist is a person who studies living organisms as a science: how they are built, how they function, how they vary, and how they have changed. The title covers laboratory work and field work alike, and it now sits beside narrower labels such as botanist, zoologist, and microbiologist. In ordinary English it is simply the agent noun for biology. The history is shorter, and slightly messier, than the Greek look of the spelling suggests.
An English formation on a new science-name
English did not inherit biologist from classical authors. It built the noun at home by attaching -ist to biology. That suffix had been English for a long time. It came through French -iste and Latin -ista from the Greek agent ending that marks a person who practices a craft or follows a method. By the nineteenth century it was the default way to name a specialist in a science.
Biology, the parent, is only a little older. Sources disagree on the first English date in the modern scientific sense. Merriam-Webster and notes based on the Oxford English Dictionary point to 1799, in the writing of the physician Thomas Beddoes. Another standard etymological dictionary dates the settled English science-word to 1819. Behind those English dates lies a Continental coinage. New Latin biologia is recorded in the eighteenth century; one historical dictionary places it in 1766. In 1802 the German naturalist Gottfried Reinhold Treviranus and the French naturalist Jean-Baptiste Lamarck both used Biologie and biologie as a name for the science of life, and they appear to have done so independently. American Heritage derives English biology from German Biologie, itself from Greek bio- plus -logia.
The agent noun followed while that science-name was still taking hold. One widely cited etymology dates biologist to 1813. The Oxford English Dictionary has historically given 1829 for an early sense; later antedating work has found biologists in a London medical oration of 1817. The first English printing is therefore not a single agreed year. It is enough to say that the word belongs to the first decades of the nineteenth century, when the science itself was still choosing its name.
The Greek pieces and an older root
Once biology is taken apart, the rest of the story is morphological rather than mysterious. Biology is bio- plus -logy. Bio- is from Greek bios, which in older usage meant a life as it is lived: a lifetime, a livelihood, a way of living. It did not, in classical Greek, do the work that English biology now does. Greek had another word, closer to organic or animal life. Biblical Greek uses bios for the span of a life, the means of living, and the manner of living. Biography still keeps that older range: a written life, not a cell culture.
The second half, -logy, is from Greek -logia, from logos, an account or reasoned discourse. The same ending built geology and theology. The reconstructed Proto-Indo-European root behind bios is *gwei-, also written *gʷeyh₃-, with the sense live. That root is not confined to the bio- spelling. English quick once meant living. Latin-derived vital and vivid belong here, and so does the zoo- of zoology. The two Greek life-words are cousins, not interchangeable labels.
From about 1800, scientific English stretched bio- to mean organic life in general. Zoo- had already been narrowed to animals as opposed to plants, so the available combining form for life as such became bio-. That is why a biologist may study plants, fungi, or microbes, even though the Greek noun at the front of the word once pointed toward a human life-course.
Related agent nouns in other languages are parallel formations on the same international science-word: French biologiste, German Biologe, Dutch bioloog, Spanish biólogo, Italian biologo, Russian биолог. They confirm the spread of the discipline-name, not a single borrowing path into English.
How the sense moved
Early English biologist did not always mean a practitioner of the modern life sciences. The first uses still leaned on the older Greek range of bios and referred to human life. Only by about 1859 is the word clearly a person skilled in, or studying, biology as a discipline. That gap of a few decades matches the time it took for biology itself to become the name of a unified science rather than a learned way of talking about lives.
English also tried a rival, biologian, attested from 1865. It did not last. French had an obsolete biologue beside biologiste. English kept the -ist form, the same pattern as chemist, botanist, and geologist, and let the meaning catch up with the laboratories.
The change in sense is the expected one. Older English already had botanist and zoologist for narrower fields. Once those fields were grouped as one science of living things, a cover term for the person who practiced it followed. Biologist filled that slot.
A discarded rival
The lost form biologian is the sharpest leftover in the record. Mid-nineteenth-century English was still shopping for a learned agent noun and had not yet decided that -ist was inevitable. The form that won is the plainer one: the new science-name plus the everyday specialist suffix. That, rather than a classical job title, is the origin of the word biologist: an English compound from the decades when scholarly life stopped meaning a biography and started meaning organisms.