It is now known that microbial genes are transferred not only vertically from a parent organism to its progeny, but also horizontally to relatives that are only distantly related, e.g., other species and other genera. This latter process is known as horizontal gene transfer. Through mechanisms such as transformation, transduction, and conjugation, genetic elements such as plasmids, transposons, integrons, and even chromosomal DNA can readily be spread from one microorganism to another. As a result, the old three-branched “tree of life” in regard to microorganisms (Figure \(\PageIndex\)) now appears to be more of a “net of life.” Metabolism is a general term describing the complex biochemistry that occurs inside of cells. Many species of bacteria are autotrophs, meaning they can create their own food source without having to eat other organisms.
However, larger eukaryotic cells have evolved different structural adaptations to enhance cellular transport. Indeed, the large size of these cells would not be possible without these adaptations. In general, cell size is limited because volume increases much more quickly than does cell surface area.
Their DNA is free in the cytoplasm in a region known as the nucleoid. Most living things composed of only one cell are microscopic and cannot matching couple suits be seen by the naked eyes. Some archaea, for instance, can survive in extreme environments, and so they are called extremophiles.
Currently, putatively the oldest biomarker records are only ~800 million years old. The nature of steranes as eukaryotic biomarkers is further complicated by the production of sterols by some bacteria. Although there is still considerable uncertainty in global eukaryote phylogeny, particularly regarding the position of the root, a rough consensus has started to emerge from the phylogenomic studies of the past two decades. The majority of eukaryotes can be placed in one of two large clades dubbed Amorphea and the Diaphoretickes, which includes plants and most algal lineages. The proposed phylogeny below includes only one group of excavates , and incorporates the recent proposal that picozoans are close relatives of rhodophytes.
The eukaryotes are usually now regarded as having emerged in the Archaea or as a sister of the Asgard archaea. This implies that there are only two domains of life, Bacteria and Archaea, with eukaryotes incorporated among archaea. Eukaryotes represent a small minority of the number of organisms, but, due to their generally much larger size, their collective global biomass is estimated to be about equal to that of prokaryotes. Eukaryotes emerged approximately 2.3–1.8 billion years ago, during the Proterozoic eon, likely as flagellated phagotrophs. Their name comes from the Greek εὖ (eu, “well” or “good”) and κάρυον (karyon, “nut” or “kernel”). Prokaryotes not only can use different sources of energy but also different sources of carbon compounds.