As we think about life on Earth and where it lives, it is clear the biological theme of unity and diversity underlies the study of biogeography. Every living thing is made up of the same molecular building blocks, but studying biogeography showcases the huge diversity of living organisms. Organisms are unified by the building blocks of life—from the largest blue whale to the tiniest bacterium, all living things are made from DNA blueprints. The knowledge stored in DNA allows life to take on all sorts of forms and live in diverse habitats.
Archaea, the prokaryotic microorganisms in the third domain of life, are famous for the diversity of extreme habitats they live in—extremely salty, hot, or acidic environments. Some archaea live in super salty environments, such as the Dead Sea and the Great Salt Lake. These organisms don't just like salt, the way people like salty nachos; they need salty conditions to survive. The creatures that live in salty habitats are called halophiles, and you could say they are endemic to salty locations.
A satellite image of the Great Salt Lake. The northern part is separated from the southern part by a railroad, which also causes the salt concentrations to be different on the two halves. The reddish color in the northern half is caused by halophiles.
Salt-loving organisms can actually be archaea, bacteria, or eukaryotes. Despite the fact that these are three different domains of life, organisms as diverse as these can live in the same extreme habitat and manage to survive. Each has to maintain the osmotic pressure within its cell walls to survive. Halophilic archaea pump sodium ions out and potassium ions into their cells, and halophilic bacteria and algae bring in organic compounds to balance out the salt. All of these organisms are unified by their ability to maintain osmostic pressure under very salty conditions that would be toxic to most other cells. We certainly don't see any Great Salt Lake octopuses, now do we?
Microorganisms can also live in a diversity of other extreme environments—super hot, super cold, acidic, toxic, radioactive and the list goes on and on. What is so crazy is that these organisms are each only one cell, but can live in diverse habitats that are toxic to all other life. The history of the Earth's climate and continents has definitely influenced organisms living in extreme environments, just as Earth's history affects biogeography of everything else alive. Scientists think that the earliest organisms living on Earth were similar to the microorganisms living in extreme habitats today. After all, early Earth was an extreme place. There is unity in the metabolisms of microbes from early Earth and their descendants today.