Palaeontology and Evolutionary Biology

Lucy

Australopithecus afarensis (Lucy) : Wiki Commons

24 November is a great day for palaeontology and evolutionary biology.  Not only was it on this  date in 1859 that Charles Darwin’s “Origin of Species,” as it is commonly known, was published (it sold out on its first day and had to be re-printed), it was also the day, in 1974, when paleoanthropologists discovered Lucy, a 3.2 million year old skeleton of Australopithecus afarensis, an extinct species of hominid.

In the broadest definition, a hominid is an erect bipedal primate that includes modern day humans and other living species of ape, as well as other extinct species such as Homo habilis. Lucy was discovered in 1974 by David Johanson, Maurice Taieb and Yves Coppens, in Hadar, Ethiopia.  She is of immense interest because so much of the skeleton was found together at one site, reducing the risk of confusion between different fossil specimens. (Modern forensic techniques make it far easier to be sure that bones are all from one or a number of different specimens, even if they are found close together, but in the early 1970’s DNA technology was nowhere near as advanced, DNA itself only being discovered in 1953.)

So far, the remains of over 300 individuals have been found, and we now know that the species lived for over 900,000 years (compared with Homo sapiens sapiens, which has so far only been around for about 195,000 years).

Reconstruction of Lucy – Australopithecus afarensis : Wiki Commons

Australopithecus had a prominently projecting bottom jaw and other ape-like facial characteristics.  The canines and molars were larger than those in modern humans, but smaller than those in modern day apes.  From close examination of the dentition of A. afarensis, it is possible to tell that the species ate fruit and other soft types of vegetation, but also harder types of food if fruit was less plentiful.  The brain case is smaller than in modern humans – Lucy’s brain would have been about a third of the size of ours.  The brain of the males averaged about 4’ 11” (151cm) tall, while that of the females averaged 3’ 5” (105cm) in height.

We can be sure that A. afarensis was able to walk in an upright posture, but what we don’t know – and there is much debate about this – is whether it habitually did this, what the gait was like or whether it walked like this all the time.  There is a wrist-locking mechanism in the species that is similar to that seen in other species of primate which may suggest that they engaged in knuckle-walking.  The fingers and toes are similar to those of modern day apes, which indicate that A. arafensis was probably adapted for tree climbing, although the species doesn’t have the ability to grasp with the big toe in the way that apes do.  The pelvis is closer in appearance and structure to humans than that of apes.  This all means that there is evidence to suggest that there are familial links to other primate species, and supports Darwin’s theory of evolution, but does not conclusively confirm it.

Charles Darwin wrote his book following the work of scientists such as Linnaeus, Lamarck and others.  Although the traditional, creationist view about how life evolved was changing at the time of writing, the new views proposed by Darwin were still treated with distrust by the established scientific community.  However, he was a respected scientist.  He had been part of an expedition on HMS Beagle, which started in 1831, and used what he learned during that voyage to build on work he had previously undertaken about natural history.  Although Darwin’s theories fell from grace from the 1880s to the 1930s, since then, they have become widely accepted in the scientific community nowadays to be the basis of evolutionary theory.

In brief, Darwin’s theory proposed that:

  1. More individuals are produced than can survive;1
  2. There is a struggle for existence, because of the disparity between the number of individuals produced in reproduction and the number that can survive;1
  3. Individuals show variation.  No two individuals are exactly the same.  Those with advantageous features have a greater chance of survival in this struggle (natural selection).  (NB.  This is not normally the case for those organisms that produce by binary fission or are just clones of each other by other means of asexual reproduction.)1
  4. As selected varieties will tend to produce offspring similar to themselves (the principle of inheritance), these varieties will become more abundant in subsequent generations.1

We now know that variation occurs as a result of sexual reproduction (half the chromosomes coming from one parent and half coming from the other), crossing over during meiosis and genetic mutation.  We know that individuals vary from each other – even siblings can appear physically very different (and, sometimes, very similar).  We also know that, excepting humans and their intervention, population sizes do remain roughly the same size, as shown by population growth curves, and that food sources, in general terms, and without human intervention, remain relatively stable over time.

A Reversible Color Polyphenism in American Peppered Moth (Biston betularia cognataria) Caterpillars. Wiki Commons

It is believed that sexual reproduction evolved as a means of combating unstable climatic conditions.  It might seem reasonable that if two parents gave birth to a number of offspring, it would be advantageous for them to all have slightly different characteristics in order to provide the widest possible chance of the genes proliferating.  However, there are other factors that come into play (for example, genetic mutation), but there is evidence to suggest that types of organism do flourish in certain conditions – we only have to look at the melanic form of Biston betularia (the peppered moth) to see this.  The original form of this moth rests on the Silver Birch tree, and other trees bearing light coloured lichen.  Its light colour provides camouflage against predators.

There has always been a small number of the melanic form of this species, however, during the Industrial Revolution, when, in the north of England, the residue from the coal fired chimneys discoloured the light barks and lichen, making them dark, the melanic form of the moth proliferated, and the original form reduced in number.  After many years, when the clean air acts were introduced, and the barks and lichen returned to their normal colours, once again the original form proliferated.  The colour of the moth is genetically controlled so surely this is natural selection in action?  It demonstrates how change can occur over time.

In general terms, evolution is thought to have progressed in the direction of simpler organisms (eg unicells) to more complex organisms (eg mammals, deciduous trees) and the simple assumption is that each more complex species is better adapted to its environment.  And whilst this may frequently be the case, this does not account for the success of organisms such as viruses and bacteria, proving that there is still much still to be learned about evolutionary biology.

The science is full of controversy and disagreement, even amongst those who do believe that evolution exists.  No doubt we will discover more as science progresses and other fossils are discovered – uncovering the knowledge in this area is an evolutionary process itself.  Darwin pointed us in the right direction; Lucy’s discoverers continued the work.

References :

1 1992, S365, Evolution A Biological and Palaeontological Approach, Book 1, Chapter 1.  Open University, Wheatons, Exeter.

Written by Susan Shirley

© Susan Shirley

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