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5

Origins of Life and Earth Prehistory

Table 5.1 The hierarchical scheme of the descriptive taxonomy of eukaryotes. Examples are given

for an individual human being and the culinary garlic

Name

Example (1)

Example (2)

Kingdom

Animalia (metazoa)

Plantae (green plants)

Phylum

Chordata

Angiospermophyta

Subphylum

Vertebrata

–

Class

Mammalia

Monocotyledonae

Order

Primates

Asparagales

Suborder

Anthropoidae

–

Superfamily

Hominoidae

–

Family

Hominidae

Alliaceae

Genus

Homo

Allium

Species

sapiens

sativum

Individual

Fred Bloggs

–

Eukaryotes are believed to have been created by the incorporation of bacteria into

an archaeon (endosymbiosis); they became the mitochondria. The endosymbiosis

of photosynthetic bacteria created chloroplasts. These new entities were much more

energy-efficient than the prokaryotes; the emergence of eukaryotes seems to have

been the key to the subsequent development of complex, multicellular life forms

(Lane 2016). Fossils of these very early creatures are of course very difficult to

come by, but recently fossils of what may have been the earliest multicellular ani-

mal, a sponge, have been found and dated to about 890 million years ago (Turner

2021). Considerably more sophisticated is Saccorhytus coronarius, dated to about

540 million years ago (Han et al. 2017). This was around the time of the “Cambrian

explosion” (Gould 1989), when we see a fantastic diversification of life forms. Table

5.1 indicates the major groupings of living and growing things.

The discovery of exoplanets has spurred interest in extraterrestrial life. The advan-

tages and ubiquity of water and carbon make it not unreasonable to suppose that

chemistries similar to that of terrestrial life could have emerged elsewhere, and we

must also be prepared to look for evidence that life may have started and then died

out, perhaps without achieving the sophistication of terrestrial life.

Problem. Devise a copiable information storage medium based on carbon but dif-

ferent from nucleic acids.

Three lineages are recognized: the archaea (represented by extremophilic prokary-

otes, formerly known as archaebacteria), the eubacteria (true bacteria, to which the

mitochondria and chloroplasts are provisionally attributed), and the eukaryotes (pos-

sessing true nuclei). The eukaryotic kingdoms are animalia (metazoa), plantae, fungi,

and protista (protozoa, single-celled organisms, including algae, diatoms, flagellates,

amoebae, etc.). The approximate numbers of species of these different kingdoms are