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The animal kingdom, the tree of life & where we come from

The word “animal” comes from the Latin word animalis, meaning “having breath.”

Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. An animal’s body plan eventually becomes fixed as it develops, although some animals undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and independently. All animals are also heterotrophs, meaning they must ingest other organisms or their products for sustenance.

Most known animal phyla (divisions) appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago. The kingdom Animalia contains approximately 35 phyla; the kingdom Plantae contains 12 divisions.

Nearly all animals undergo some form of sexual reproduction. They have a few specialized reproductive cells, which undergo meiosis to produce smaller, motile spermatozoa or larger, non-motile ova. These fuse to form zygotes, which develop into new individuals.

Many animals are also capable of asexual reproduction. This may take place through parthenogenesis, where fertile eggs are produced without mating, budding, or fragmentation.

Aristotle divided the living world between animals and plants, and this was followed by Carolus Linnaeus (Carl von Linné), in the first hierarchical classification. Since then biologists have begun emphasizing evolutionary relationships, and so these groups have been restricted somewhat. For instance, microscopic protozoa were originally considered animals because they move, but are now treated separately.

Charles Darwin, in his seminal book, The Origin of Species, proposed that phylogeny, the evolutionary relatedness among species through time, was expressible as a metaphor he termed the Tree of Life. The modern development of this idea is called the Phylogenetic tree.

Using the Phylogenetic tree, scientist have been able to propose a theory of the interrelatedness of the various forms of life from the lowest forms of Eukaryotes (organisms with complex cell structures encased in a membrane) and Prokaryotes (bacteria and archaea), to the animal species that are extinct and those we know today. Animals can be categorised under:

  • Vertebrates, i.e. those that have  a backbone or spinal column, including mammals, birds, reptiles, amphibians, and fishes and,
  • Invertebrates, i.e. an animal lacking a backbone, such as an arthropod, mollusk, annelid, coelenterate, etc. Invertebrates comprise 95 percent of animal species and about 30 different phyla.

Mammals (from the Latin word mamma meaning”breast”, are members of a class of air-breathing vertebrate animals characterised by the production of heat from internal means (endothermy), hair, three middle ear bones, and mammary glands functional in mothers with young. Most mammals also possess sweat glands and specialised teeth, and the largest group of mammals, the placentals, have a placenta which feeds the offspring during gestation. The mammalian brain, with its characteristic neocortex, regulates endothermic and circulatory systems, the latter featuring red blood cells lacking nuclei and a large four-chambered heart maintaining the very high metabolism rate they have. Mammals range in size from the 30–40 millimeter (1- to 1.5-inch) Bumblebee Bat to the 33-meter (108-foot) Blue Whale.

Primates are mammals that arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment. All but a few primate species remain at least partly arboreal (tree of forest dwelling). Humans are of course, primates.

The Order Primates has traditionally been divided into two main groupings:

  • Prosimians and anthropoids (simians). Prosimians have characteristics more like those of the earliest primates, and include the lemurs of Madagascar, lorisiforms and tarsiers.
  • Simians include the monkeys, apes and humans. More recently, taxonomists have preferred to split primates into the suborder Strepsirrhini, or curly-nosed primates, consisting of non-tarsier prosimians, and the suborder Haplorhini, or dry-nosed primates, consisting of tarsiers and the simians.

Simians are divided into two groups:

  • Platyrrhine (“flat nosed”) or New World monkeys of South and Central America and
  • Catarrhine (narrow nosed) monkeys and apes of Africa and southeastern Asia.

New World monkeys include the capuchin, howler and squirrel monkeys; catarrhines consist of Old World monkeys (such as baboons and macaques), gibbons and great apes. Humans are the only extant catarrhines that have spread successfully outside of Africa, South Asia, and East Asia, although fossil evidence shows many other species were formerly present in Europe.

With the exception of humans, who inhabit every continent, most primates live in tropical or subtropical regions of the Americas, Africa and Asia. Primates range in size from Madame Berthe’s mouse lemur, which weighs only 30 grams (1 ounce), to the mountain gorilla, weighing 200 kilograms (440 lbs). According to fossil evidence, the primitive ancestors of primates may have existed in the late Cretaceous period around 65 million years ago; the oldest known primate is the Late Paleocene Plesiadapis, c. 55–58 million years ago. Molecular clock studies suggest that the primate branch may be even older, originating in the mid-Cretaceous period around 85 mya.

Apes are Old World anthropoid mammals, more specifically a clade of tailless catarrhine primates, belonging to the biological superfamily Hominoidea. Humans are apes.

Apes are native to Africa and South-east Asia, although in relatively recent times humans have spread all over the world. Apes are the world’s largest primates; the orangutan, an ape, is the world’s largest living arboreal animal. Hominoids are traditionally forest dwellers, although chimpanzees may range into savanna, and the extinct australopithecines are famous for being savanna inhabitants, inferred from their morphology. Humans inhabit almost every terrestrial habitat.

Hominoidea contains two families of living (extant) species:

  • Hylobatidae consists of four genera and sixteen species of gibbon, including the lar gibbon and the siamang. They are commonly referred to as lesser apes.
  • Hominidae consists of orangutans, gorillas, common chimpanzees, bonobos and humans. Alternatively, the hominidae family are collectively described as the great apes.

Members of the superfamily are called hominoids (not to be confused with “hominids” or “hominins”). Hominids (or Hominidae) are also known as great apes with four extant genera (living species): Common chimpanzees and bonobos (Pan), gorillas (Gorilla), humans (Homo), and orangutans (Pongo).

Through DNA comparison, scientists believe the Pan/Homo divergence occurred between 5.4 and 6.3 million years ago, after an unusual process of speciation that ranged over 4 million years. In other words, the great apes share a common ancestor. If you go further back still, great apes and lesser apes share a common origin, and further back, primates, mammals, vertebrates and invertebrates, all share a common origin.

Homo is the genus that includes modern humans and species closely related to them. The genus is estimated to be about 2.3 to 2.4 million years old, evolving from australopithecine ancestors with the appearance of Homo habilis. Specifically, H. habilis is assumed to be the direct descendant of Australopithecus garhi which lived about 2.5 million years ago.

Humans (known taxonomically as Homo sapiens, Latin for “wise man” or “knowing man”) are the only living species in the Homo genus. Anatomically modern humans originated in Africa about 200,000 years ago, reaching full behavioral modernity around 50,000 years ago.

Modern humans branched out and colonised the rest of the world from an East African origin starting about 50,000 years ago.

Kenyans are a group of humans living in the geographic and political boundaries established by European monarchies slightly over 100 years ago and constitute several language and culture groups that migrated into and around the region over recent millennia, settling and exploiting diverse enviroments.

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