Evidence of Evolution

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Key Components

Before we begin, here are some terms which you should familiarise yourself with.

  • Half Life: the time taken for half of a radioactive element to decay
  • Palaeontology: the study of ancient life represented by fossils
  • Strata: layers of rock (singular is stratum)


Fossils are the preserved remains or traces of any organism from the remote past. They include, bones, teeth, shells, leaves, footprints, tooth marks and tracks.
Types of fossils include:

  • Impression fossils are formed when an organism decays leaving the impression of the external or internal surface. If the vacant space of the mould is later filled with a foreign material, it forms a 3D sculpture and is called a cast fossil
  • Mineralised fossils are formed when the organic material is replaced by minerals from surrounding sediments
  • Trace fossils are not a part of any organism, they are the preserved evidence of an organism’s activity or behaviour
  • Transitional fossils represent the intermediate stages of evolution between distinctly separate species
  • Mummified organisms are those that have been trapped in a substance under conditions that reduce decay. For example, insects trapped in amber and animals frozen in ice

A Fossil

Relative Dating

Relative dating is the science of determining the relative age of an object in relation to a stratum or a fossil. Stratigraphy is the study of the relative positions of the rock strata, some of which contain fossils. The law of superposition states that the lowest stratum is the oldest and the upper strata are progressively younger. The age is estimated relative to the known age of the strata above and below the layer in which the fossil is found

Index fossils are distinctive, abundant, widely distributed geographically and that lived for short or well-defined periods. Therefore, when found, they can be used to date other fossils found in the same rock layer

Relative dating is difficult in areas where strata have been eroded/moved, altering the original sequence of stratum. Instead, the age of strata can be determined by the presence of igneous rocks dated by absolute dating or by index fossils


Absolute Dating

Absolute dating is based on the decay of certain radioactive elements – the rates at which they decay at are known.

Potassium-Argon dating is useful in measuring the ages of igneous rocks. This is because the high temperatures of molten rock prevent the accumulation of argon, but once the rock has solidified into an igneous rock, the rock clock starts and potassium decays into argon which accumulates over time

Carbon-14 dating measures the ratios of C12 to C14. It is used to determine the age of organic substances, e.g. after the death of an organism, it will stop ingesting carbon, and therefore C14 will continually decay over time

Technique Timeframe (Years) Materials
Carbon-14 1000-60,000 Wood, shells, carbon based
Potassium-Argon 500,000+ Volcanic rocks and minerals


Fossilisation is the preservation of the hardened remains or traces of organisms in sedimentary rock

Mineralised fossils form when an organism is rapidly buried and compressed under layers of sediment – sand, silt or clay. When the organic material in the bones of an animal gradually decays, the bones become porous allowing minerals in the surrounding sediment to seep in and gradually fill in the pores – forming a fossil. Impression fossils form when a rapidly buried organism slowly decays leaving an impression in the rock

Conditions for fossilisation include:

  • Preservation of remains, Rapid burial
  • Protection against scavenging, erosion and environmental damage
  • Low oxygen to protect against decay and bacteria
  • High pressure to promote mineralisation of remains
  • Hard body parts, bones, teeth and shells

Structural Morphology

  • Homologous structures may serve different functions, but are fundamentally and morphologically similar, indicating that the species shared a common ancestor - different function, but same structure
  • Analogous structures carry out similar functions that evolved in species that don’t necessarily share a common ancestor, due to facing similar selection pressures – same function but different structure
  • Vestigial structures are structures remaining in an organism which had a role in the ancestral species but are now functionally useless

Developmental Biology

  • Developmental biology is the study of the processes that result in the growth and development of organisms
  • Comparative embryology is where organisms that share a common ancestor will show similar embryological development. For example, tails are present in most adult vertebrates, but not adult foetuses and human babies, however at one stage, we do have a primitive tail in our embryonic development

Comparative Embryology