CHAPTER 26 | ASTROBIOLOGY: LIFE ON OTHER WORLDS 591
environment is called an extremophile. Maybe you have friends
like that.
Linguists can fi gure out the vocabulary of the long-vanished
Indo-European language by comparing words in modern lan-
guages such as English, Spanish, Russian, Greek, and Hindi that
evolved from it. In much the same way, biologists can work out the
DNA sequences of ancient species by comparing the sequences of
their present-day descendants. Th is type of analysis indicates that
the creature that was ancestor to all life on Earth today resembled
present-day single-celled organisms known as archaea, especially
the extremophile types of archaea that are tolerant of high tem-
peratures, called thermophiles (Figure 26-8b). Some biologists
think this is evidence that life began near volcanic vents on the seaExtremophiles
Scientists on Earth are fi nding life in places previously judged
inhospitable, such as the bottoms of ice-covered lakes in
Antarctica, far underground inside solid rock, and among the
cinders at the summits of extinct volcanoes (■ Figure 26-8a). An
organism that can survive and even thrive in an extreme
■ Figure 26-7
Complex life has developed on Earth only recently. If the entire history of
Earth were represented in a time line (left), you have to examine the end of
the line closely to see details such as life leaving the oceans and dinosaurs
appearing. The age of humans would still be only a thin line at the top of
your diagram. If the history of Earth were a year-long videotape, humans
would not appear until the last hours of December 31.
Billion years ago Million years agoFirst anthropoidsAge of humansLife on landFormation of EarthCambrian periodDecNovOctSeptAugJulyJuneMayAprilMarchFebJanFirst horsesFirst flowering
plantsLast
trilobitesCoal-forming
forestsFirst forestsFirst life on landAge of
marine
invertebratesAge of
fishesAge of
amphibiansAge of
reptiles
(dinosaurs)Age of
mammalsOcean life onlyFirst birdsFirst primitive
mammalsCambrian
explosionPrecambrian
periodOrigin of life44.632100100200300400500600CretaceousJurassicTriassicPermianPennsylvanianMississippianDevonianSilurianOrdovicianCambrianPre-
cambrianTertiary