The Fungi constitute a large and diverse group of organisms. Until the 1960s, fungi were considered members of the plant kingdom. With the advent of the five-kingdom system of biological classification, fungi were assembled into a single kingdom because of their similar ecological roles as primary decomposers of organic matter and their similar anatomical and biochemical features. Recent studies that compare the sequence of amino acids in proteins from fungi, plants, and animals now indicate that fungi share a closer evolutionary relationship to animals than to plants.
The evolutionary ancestry and relationships of the different fungi are not well understood. There are few fossils of fungi, presumably because their relatively soft tissues are not well preserved. There is some fossil evidence that they existed in the Precambrian era (over four billion years ago), although identification of these very early fossils is uncertain. There is definite fossil evidence for fungi in the lower Devonian (about 400 million years ago) period. Fossils of all the major groups of fungi are found in the Carboniferous period (about 300 million years ago). By the late Tertiary period (about 20 million years ago), the fossil record shows a rapid and divergent evolution of fungi. Many of the Tertiary fossils of fungi are similar to existing species.
In addition to the incomplete fossil record, there are at least two other reasons why the evolutionary relationships of fungi are not well-known: they tend to have simple morphologies and they lack embryos which follow a definite sequence of developmental stages. Biologists often use these two features to determine evolutionary relationships of animals and plants.
Another complication is that many fungi presumably evolved similar morphologies by convergent evolution. That is, unrelated species may share a common morphology because they have been subjected to similar selective pressures.
The relatively new technique of molecular systematics is particularly useful in the study of the evolution of fungi. This technique compares the sequence of DNA segments of different species to determine evolutionary relationships. One important finding from this new technique is that the plasmodial slime molds, cellular slime molds, and water molds are only distantly related to the true fungi (the taxonomic groups considered here). Evolutionary relationships among organisms also are being studied by comparing the sequences of ribosomal RNAs and transfer RNAs from different organisms. Although these RNAs are similar to DNA, they have structural roles in cells, rather than coding for proteins as do DNA and messenger RNAs. Therefore, the sequences of ribosomal and transfer RNAs tend to be more conserved through evolution. Comparison of these RNAs among fungi, plants, and animals also suggests that fungi are more closely related to animals than to plants. Future work in molecular systematics is expected to tell us more about the evolution and relationships of the fungi.