What is Life?_Concept_2

Subject Facts

classification

To get from ‘It is a living thing' to 'It is a human' requires at least seven levels of classification. The process of classification progressive, going through a sequence of levels:

Kingdom - this describes the cell structure and organisation (and for multicellular describes how energy is obtained). The animal kingdom is defined as: multicellular, non-photosynthetic organisms that coordinate using nerves.

Phylum — the differentiation of different groups at this level and beyond depends on the content of the particular kingdom, Humans belong to the phvlum chordates, which have a nerve cord and brain within a spine and skull.

Class - humans belong to the class known as mammals, which have milk-bearing glands for their young and have internal fertilisation.

Order- the primates, the order to which humans belong are those animals possessing an opposable thumb.

Family- humans belong to the family of anthropoids, which contains monkeys and apes but not lemurs.

Genus- these are closely similar in appearance. Apart from human beings, all members of the genus Homo are now extinct.

Species — members of a species are so closely related that breeding between them is possible. In this case, ail human types or races belong to the species Homo sapiens.

The five kingdoms

Children at KS1 and KS2 only need to know much about two of the five kingdoms: plants and animals. Plants are defined as those multicellular, photosynthetic organisms whose cell walls contain cellulose — that is, green plants with clearly defined leaves or stems (such as trees, grass and moss). For a definition of animals see above.

Two other kingdoms that deserve a mention are fungi and prokaryotes. The latter are usually referred to as 'micro-organisms' or 'bacteria'. Fungi are decomposers: they gain their nutrients by assisting with the decay of dead organic matter. They perform this recycling task alongside certain types of bacteria (a large group within the prokaryote kingdom), which also gain their nutrients from decaying organic materials.

Prokaryotes

The main difference between the kingdoms of fungi and prokaryotes is that fungi tend to be multicellular organisms,

whereas prokaryotes (being a much simpler form of life and lacking a nucleus to their cells) are not, Another difference is that prokaryotes are not all decomposers — there are many different forms, classified according to the way they gain their carbon (from which all living things on Earth are made) and energy.

Autotrophic bacteria obtain their carbon from carbon dioxide, heterotrophic bacteria from organic material (compounds containing carbon. hydrogen and oxygen). The energy that they use to turn the carbon into replicas of themselves can be obtained either from light (the Sun) or from chemicals, Those that use light (photoautotrophs) include bacteria which live in the ocean and oxygenate the water, and also provide a foundation for the food chain in the seas. Some bacteria that use chemicals as a source of energy (chemoheterotrophs) can use organic material as both an energy and a carbon source, These Variations mean that bacteria, which are the most ancient form of life on Earth, are found almost everywhere. They' perform a huge range of functions in human life: some cause illness, while others are a source of antibiotics and other life-preserving drugs; some are important in processing food, others in processing effluent.

The fifth kingdom

Possibly the least well-known of the kingdoms is the collection of living things called 'protoctista'. These differ from prokaryotes in one very significant respect: their cells have a nucleus, This means that they are one step up on the evolutionary ladder; but they are still just singe cells (or simple collections of similar cells). They rely on light as a source of energy, but they have not developed specialised cells (to form roots leaves or stems) this is what separates them from plants.

The classification of the protoctista kingdom is made more difficult by one of its best known groups, algae, being shared between the prokaryote and protoctista kingdoms. Cyanobacteria (commonly known as 'blue-green algae') lack a nucleus to their cells, and so are a member of the prokaryote kingdom, Green, red and brown algae have nuclei in their cells, and so belong to the protoctista kingdom. These latter three forms of algae are commonly known as seaweed kelp and pond scum,

There is an interesting little group of living things called lichens. These represent a symbiotic relationship between two types of organism: an alga and a fungus. The alga produces a type of carbohydrate which the fungus uses as a foodstuff; in return, the fungus protects the alga from the extremes of the environment by retaining moisture. As an example of teamwork, it seems to work well: there are 15000 known types of lichen.

Plants and animals

Now we come to the kingdoms most frequently used to represent living things, particularly during the primary phase of education. Plants are multicellular organisms that photosynthesise (gain energy from light), and whose cell walls are made rigid by containing cellulose Animals are also multicellular organisms; but unlike plants, they do not photosynthesise. This description would place them in the same group as fungi - so two additional characteristics, the use of egg production in reproduction and coordination through specialised nerve cells complete the distinction.

Plants

Plants are divided into six distinct groupings, or phyla. The most familiar is the phylum of 'flowering plants'. Discussed elsewhere. Ferns and gymnosperms (conifers and palms) are the other 'vascular' plant groups. Vascular plants are those with internal transport systems' for water and nutrients. Non-vascular plants make up the other three phyla: mosses, liverworts and hornworts. These plants do not have long roots which they can send down into the ground in search of water, and so can only grow on moist surfaces where they are in direct contact with water.

Ferns are some of the most ancient forms of plant life. They dominated the landscape in the Carboniferous period (359 million years: ago), and formed the major part of the diet of the first dinosaurs. Most ferns are recognisable by their thin, feathery leaves or fronds — but only in a certain phase of their lives. Ferns have a two-stage life cycle. Firstly, the recognisable plant produces spores by simple cell division. These spores germinate and develop into plants that often have a small, flat heart-shaped structure containing male and female parts. In the presence of water; these parts combine to produce a fertilised seed, which grows into the familiar fern plant.

Gymnosperms (literally 'naked-seed' plants), including conifers and cycads (palms)! have also existed since ancient times. Cycads were the dominant form of plant in the Jurassic period (200-145 million years ago). These plants are thought to have evolved from ferns. They tend to be woody (trees and bushes), and they grow directly from seeds. The seeds grow in protective seed cones: structures of woody scales that respond when ripe, to the moisture content of the air; so that the seeds are released when the growing conditions are most favourable. Although most of the cycads (the most ancient form of gymnosperm) have become extinct, the conifers are still going strong.

Animals

Figure 1 shows a 'family tree' or branching classification of the major types of animal. The animal kingdom contains over 30 phyla — although most of them are fairly obscure marine-dwelling forms, There are at least 12 different phyla used to classify what a non-biologist would call 'worms’ — but any one of these phyla has less in common with the others than we do with a goldfish. The three animal phyla most commonly referred to are

Molluscs - unsegmented animals with a muscular foot and often with a shell (snails, slugs, shellfish and others).

Arthropods — segmented animals with a jointed exoskeleton (insects, spiders, crabs and others).

Chordates - animals with an internal bony skeleton, a skull-encased brain and a nerve cord (fish, amphibians, reptiles, birds, mammals).

The diversity of species within each phylum is highly variable. At one extreme, there is a microscopic marine animal so different from anything else that its species has a phylum of its own (called placozoa or Trichoplax adherens to friends though clearly it doesn't have many). At the other extreme. there are over one million species of arthropod (by comparison, there are only 45,000 species of chordate). Many of the marine animals can only be distinguished from plants because they don't photosynthesise. Sponges, for example, are a very basic animal life form; they lack any distinct internal organs, rely on water flow for feeding and movement, and remain attached to a rock for most of their life cycle. Interestingly if you popped a sponge through a blender then poured it back into sea water; it would reform into a sponge. Actually, if you blended different species of sponge they would find their own type and reform and we think that 'sawing a lady in half' is a pretty impressive trick.

The terms 'vertebrate’ and ^‘invertebrate' are commonly used when describing animals, but are not really accurate enough for biological classification. Some marine animals could be placed in both categories at different times in their life cycle. Using the term ‘with/without’ a central nerve cord would be slightly more accurate (not all spinal cords are surrounded by a backbone).

Arthropods

This phylum is subdivided into five main classes, according to the: number of body segments and legs (and where the legs are attached). At arthropods have segmented bodies with jointed legs. the biggest class of arthropods are the insects, which have more species than all other forms of animal combined. Other arthropods include crustaceans (lobsters and crabs), arachnids (spiders, mites, scorpions), centipedes and millipedes. This phylum represents some of the oldest animal forms that have specialised body parts. In evolutionary terms, they were the first to develop limbs, wings, heads, eyes and a few other useful little bits. They are also notable for the metamorphosis (change in form) that some species go through during their life cycles.

Chordates

This phylum, to which humans belong, can cause quite a few difficulties in terms of classification. Because it contains the animals that are most closely related to us, this phylum has been subject to the greatest level of study over the years — resulting in the greatest degree of reclassification as definitions have become more precise. Due to their familiarity, many of the terms have slipped into common usage, leading to double meanings for some of the key vocabulary. Fish, for example, do not make up a distinct scientific class, but rather three classes.

The first class of fish, chondrichthyes (cartilaginous fish), include fish that have a cartilage skeleton, fleshy fins and a mouth that is backward of a snout (ventral).The second class, osteichthys (bony fish), have bony skeletons, fins supported by bony spines and a mouth that comes at the very front of the body. The third class, agnatha, are the jawless fish (such as eels) which are generally long and thin and have sucker-like mouth openings with tongues.

The first two classes represent the types that children are most like}y to be familiar with, an example of each being the great white shark (cartilaginous) and the mackerel (bony). Bony fish are more highly evolved, having developed a swim bladder which means that they do not need to keep swimming to avoid sinking. The swim bladder eventually evolved into lungs for air breathing as fish evolved as the ancestors of reptiles, amphibians, birds and mammals. Fish were the dominant and most evolved form of animal life during the Devonian period (416—359 million years ago).

Amphibians

Apart from some forms of lungfish that were able to survive on land for short periods of time, the amphibians (such as frogs, newts and salamanders) were the first class of chordate to be able to breathe air for extended periods. They are defined by the way th.at they metamorphose from gill-breathing young to lung-breathing adults, usually only returning to the water to breed. Their numbers have declined dramatically since their heyday during the Carboniferous period (359-290 million years ago). Currently they are the class of animal that is decreasing in numbers most rapidly, with significant numbers of deformed adults (with extra limbs and so on) being discovered, possibly as a result of pollution.

Reptiles

Reptiles developed from amphibians around the end of the

Carboniferous period. They continued as the dominant life form particularly the branch that evolved into the dinosaurs — until the end of the Cretaceous period (65 million years ago), a period of over 200 million years. However, they were subject to several mass extinctions during the Cretaceous period, the last leaving just four of the 23 orders known to have existed — the dinosaurs being the most famous casualty. Lizards, snakes, turtles, tortoises and the crocodilians are now the main representatives of this class.

The reptiles' great advance from the amphibians was the hardened shell of their eggs, which allowed reproduction out of water to take place and so removed the need for a metamorphosis from infant to adult. For added protection, they have scaly skin. It should be noted that not all reptiles lay eggs: some give birth to live young, though they do not feed them by producing milk like mammals.

Mammals and birds

These two classes are distinct from the others in this phylum in that they are able to regulate their own body temperatures. Although they are known as 'warm-blooded', this is not very precise (nor is. 'cold-blooded' for the others), as the blood of all chordates tends to be quite warm. The better term is endotherm which means that they control their body temperature from within. The other chordate classes are ectotherms, which means that they rely on the environment to regulate their body temperature.

Mammals are distinct: from any other form in that the female has mammary (milk bearing) glands with which to rear its young. They evolved slowly from reptiles, becoming a separate class about 200 million years ago: but it wasn't until the extinction of most reptilians (about 65 million years ago) that they began to prosper. There are three mammalian sub-classes, based upon the mode of reproduction. The monotremes are representative of the most primitive form of mammal the duck-billed platypus is a well-known example. Next came the marsupials, which give birth to partially developed young; and then the placentals, which produce live and immediately viable offspring to be fed from the mammary glands. The mammalian class will be the main focus elsewhere.

Bids are thought to have evolved about 150 million years ago from dinosaurs, along with many species that flourished alongside the dinosaurs during the late Cretaceous period. Most of these died out at the same time as the dinosaurs; but the bird class was able to re-establish itself, and most current families of bird can be traced back about 50 million years. Birds ate egg-laying endotherms with one pair of wings instead of front legs. Their bodies are covered in feathers: lightweight structures with a hollow stem and fine strands. These are thought to have evolved from scales.

Birds have turned out to be particularly difficult to classify. Those that have been assigned to the same groups according to external features and habitat have not been confirmed as near-relatives by DNA sampling. For example, the vultures of the Americas, according to DNA analysis, appear to be more closely related to storks than to African vultures. Overall about 25 orders of bird have been defined to distinguish such types as swimming birds, wading birds, perching birds and flightless birds.

Humans

All humans belong to the same species of mammal: they can interbreed. It is very likely that humans evolved in a single place and then, over a period of hundreds of thousands of years, spread themselves across the Earth. Ice ages (glacial dislocations) and changing sea levels were the likely causes of various groups of humans becoming isolated from others, leading to the development of races in different parts of the world. It is quite possible, with mass transport now freely available, that distinct human races have disappeared by the next millennium.