The 20th century will probably be remembered as the ‘Golden Age’ of discoveries in the field of human evolution but we are only a very small part of the story of life on Earth. Many weird and wonderful creatures have lived and become extinct on the path from the origin of life to the present day diversity. Here are our 10 greatest discoveries in evolution.
1 – Extinction of the Dinosaurs
There have been several mass extinctions throughout geological time, one of which saw the non-avian dinosaurs disappear from the face of the Earth at the end of the Mesozoic period. What caused this was not clear. But in 1980, a plausible explanation began to appear. Walter Alvarez and his team were working in Italy and discovered a thin layer of clay marking the K-T (Cretaceous-Palaeogene) boundary which marks the Mesozoic mass extinction event.
This layer was found in many locations worldwide and contained an unusually high concentration of the element iriduim. On Earth, it is rare but it is more abundant in space. This led Alvarez to suggest the impact of an asteroid had been the cause.
So the hunt was on to find a suitable crater which eventually turned up in Mexico. The Chicxulub crater was a massive impact structure about 125 miles diameter and the right age. It had been hard to spot as it was well-buried, half on land and half underwater.
It is still not certain that this was definitely the impact that caused the extinction; some say there could have been multiple impacts from a single object that had broken into several fragments.
2 – Identification of the Dinosaurs
Dinosaurs have been known for thousands of years but it wasn’t until the start of the 19th century that palaeontologists started to understand where they came from. The original thoughts were that they were giant versions of living lizards.
Eminent English palaeontologist, Sir Richard Owen coinied the name ‘Dinosauria’ (fearfully great lizard) as he recognised Iguanodon, Megalosaurus, and Hylaeosaurus as being from the same distinct group of creatures.
From the mid-19th century, perhaps as a result of the captivating ideas of these gigantic creatures, palaeontology took off as a science. The scientific community was finally starting to seriously consider life on Earth. Prior to that it had been simply fossil collecting with little effort made to classify finds. Since the idea of natural selection and evolution were many decades in the future, that is hardly surprising.
The big dinosaurs dominated the Earth for about 135 million years before the mass extinction put an end to the non-avian species. Their demise allowed other creatures to evolve and had they still been around today, people would not!
3 – Primordial Soup.
The ‘Holy Grail’ of evolution is to find out how life on Earth started in the first place. In 1953, Prof. Harold Urey and his student, Stanley Miller, set up an experiment to test a theory proposed in the 1920s by a Russian biologist (Oparin) and English polymath (Haldane) – the theory of the ‘primordial soup’. They proposed UV radiation provided the energy to convert chemicals in the Earth’s early atmosphere into the first organic compounds which then dissolved in the oceans and from which, life developed.
They used a system of reaction vessels through which the gases believed to form the atmosphere of the early Earth were circulated. Also in the system was some water that was heated to produce water vapour and a pair of electrodes to provide the energy for any chemical reactions. After week or so the water was analysed and several amino acids were found, supporting the primordial soup theory.
As with all theories, there are unresolved questions which drive scientific knowledge forward as new discoveries are made.
In 2010, Biochemist Nick Lane of UCL speculated that alkaline hydrothermal vents could have provided the impetus for life to arise on Earth. These were discovered around the turn of the 21st century and their geochemisty bears certain similarities to biochemistry of cells. He believes that a honeycomb of microscopic natural caverns within the structures created by these vents act as catalysts. The energy comes from a geochemical gradient of protons over a membrane, exactly the mechanism used in living cells.
Which of these will ultimately deserve its place in the 10 greatest discoveries in evolution remains to be seen …
4 – New Forms of Life Found
1977 saw the discovery of the ‘black smokers’ – hydrothermal vents on the deep sea bed. At the time, Robert Ballard was part of a team who had sent a deep sea exploration vehicle down to explore the Pacific ocean bed. The instruments detected a temperature spike and photographs taken at the time of the spike revealed the vents. They resembled permanently spouting geysers, black with mineral content, spewing superheated water into the ocean.
But what was most intriguing was that the area near the vents was warm enough to support a healthy community of never-before-seen organisms. These had evolved in this incredibly toxic environment and did not seem related to other life in the sea. From eyeless fish-like creatures to tube worms 3m in length with human-like blood, this new fauna astonished the world.
So why has this made ito onto our list of 10 greatest discoveries in evolution? They found bacteria that were able to duplicate photosynthesis, but without the need for sunlight – chemosynthesis. This challenged existing theories about how life arose and it means that life elsewhere in the Solar System cannot be ruled out.
5 – The Burgess Shale
There are various tales about how the Burgess shales were discovered by the head of the Smithsonian, Charles Walcott. His daughter, writing 45 years later even attributes the discovery to her mother (Walcott took the whole family on his field excursions).
So what are the Burgess shales and why are they important in evolutionary terms?
A rare set of conditions allowed these animals to be preserved perfectly 505 million years ago. Some of the fossils have traces of their soft parts preserved which is one of the reasons for their importance. This incredible preservation – with amazingly fine details – tell paleontologists much more about what the ancient animals looked like and how they lived. What has been learnt from these fossils can then be applied to others.
The fossils have been systematically observed, recorded and classified since the 1960s. Over 60,000 have been recovered from this small outcrop with 170 individual species known. The major phyla (types of animals) are represented in the Burgess Shale, plus others that cannot be placed in the current classification system.
Their age places them in the Cambrian period shortly (geologically speaking) after the astonishing burst of biodiversity called the ‘Cambrian explosion’. During this 20 million year period, life went from single celled to enormous complexity in the blink of a geological eye. There is no known reason for this ‘explosion’ but these fossils underline the Cambrian biodiversity.
6 – Classification and Naming
More of a development than a discovery, this is on our greatest discoveries list because without it, the study of evolution would have been a lot more difficult. Previous naturalists and botanists had been using a form of nomenclature to identify individual species but these were polynomial (i.e. having more than 3 parts), unwieldy and descriptive.
Swedish botanist, Carl Linnaeus, realised that this was too complex and could lead to confusion. His master stroke was to introduce binomial naming. He realised that the name would be a specific label for a species so the name didn’t need to be descriptive. His work, Systema Naturae was expanded over the years and became more and more popular in scientific circles of the 18th century. In England, the latin version was translated by the Lichfield Botanical Society, established by Erasmus Darwin who championed the system in the UK.
Some elements of the original system are still used but 21st century classification includes DNA, chemical analysis and where organisms fit on the tree of life.
7 – Natural Selection, the Backbone of Evolution
What an incredible back-story there is with Charles Darwin and natural selection, it’s far too long to include it all here. It was a long journey to enlightenment with the pieces coming together over a period of 35 years or so.
Charles Darwin bought into a long voyage as the companion for captain Robert FitzRoy. The ship, the Beagle, was to explore coastal waters in the southern hemisphere. Darwin wasn’t the official naturalist which enabled him to retain ownership of his observations and specimens, including those from the Galapagos Islands.
He was more of a geologist than naturalist. The captain gave him a copy of Lyell’s Principles of geology and after making his own observations, he wholeheartedly embraced Lyell’s idea of slow geological change over millions of years, a necessary timescale to allow for evolution.
Many years later, studying the observations he had made in the Galapagos Islands, Darwin became convinced that the species of bird he had recorded were in fact all finches. The idea struck him they had somehow ‘transmuted’ from one into another but there was no mechanism to drive this.
In October 1838 he read Malthus’ An Essay on the Principle of Population in which Malthus showed how overpopulation would lead to a lack of food creating population collapse through starvation. He had already read Candolle’s Warring of the Species which described biological competition; putting the two ideas together provided his mechanism.
Darwin continued to develop his thory, arguing that favourable adaptations would allow a species to breed to its full potential but unfavourable ones would not, thereby causing extinction of the species. He finally published his book the Origin of Species in 1859.
8 – Lucy
Much to the derision of the popular press, Darwinism suggested that humans had evolved from African apes. Proving it was another thing altogether and scientists started looking for the missing link between apes and humans. In 1974, American archaeologist Donald Johanson and his team unearthed fragments of bone in Ethiopia. These belonged to a single individual who subsequently became known to the world as Lucy.
Although only 40% complete, it was possible to determine that Lucy was a bipedal hominid i.e. a human ancestor who walked upright. Careful measurement of bone lengths, angles and sizes indicate that she had some human-like and some ape-like features. Her species was classified as Australopithecus afarensis and other finds confirmed they walked upright.
Accurate dating of the bones in the 1990s confirmed that she lived around 3 million years ago, making her the oldest hominid known. But she wasn’t the ‘missing link’ of Darwinism as her species became extinct 2.9 million years ago.
So why is Lucy one of the greatest discoveries in biology? She showed clearly that evolution is not linear and is more bush-like with many branches that lead to dead ends.
9 – Laetoli Footprints
This discovery was made by British paleoanthropologist Mary Leakey in Tanzania. The footprints appear to belong to three individuals who crossed a muddy area made from fine volcanic ash not long before a further eruption buried the prints, preserving them perfectly.
They have been dated at about 3.7 million years old, several hundred thousand years before Lucy. It is not certain but has been suggested that they could have been made by early Australopithecus afarensis individuals.
What makes this discovery so important is that it shows that bipedalism in human ancestors preceded the development of a larger brain size and that early human ancestors appeared to be sociable.
10 – Toumai Skull
Found in 2001/2 in Chad, these skull fragments were dated at 7 million years old, pushing the boundaries of hominid evolution back significantly further than before. It has thrown the evolution of early humankind into disarray as there are various options and unasnswered questions most notably was it bipedal and is it an anscestor of chimps, humans, both or neither?
Whatever the truth turns out, it has prompted the scientific community to question existing finds and links. Since so many hominid evolutionary branches have led to extinction, it begs the question why is homo sapiens still around? Survival of a species is more than just adaptation. As we have seen with the life forms of the Mesozoic era, it is down to luck too. Perhaps as we humans understand more about life and the universe, that knowledge could help us from becoming yet another dead end on the Earth’s tree of life.