I. Discovery of the Earliest Complex Life Forms
For a long time it was thought that complex life forms, which can be seen without the aid of a microscope, started to evolve during the Cambrian period, which lasted from about 570 million years ago to about 505 million years ago. Fossils from that period have been found worldwide in marine sedimentary rock formed at that time. The fossils are perhaps best preserved in the Burgess Shale in British Columbia, a western province of Canada. Although these life forms may seem strange, some of them showed the basic structure of living creatures today. Among them are forerunners of vertebrates and some marine creatures.
In 1947, a geologist who worked for the Australian government, R.C. Sprigg, made a startling discovery. In the Ediacara Hills in southern Australia, Sprigg discovered some strange fossils, looking somewhat like jellyfish. The fossils were embedded in strata that were more than 600 million years old. Here was evidence of complex life forms earlier than Cambrian times. Since that finding, similar fossils from what we call the Precambrian period have been found at more than thirty sites around the world. Because these early fossils were first discovered in the Ediacara Hills, they are referred to as Ediacaran life forms.
Using radioactive dating methods, scientists studied these fossils. Their work showed that Edicaran life flourished between 670 and 543 million years ago. They were soft-bodied marine creatures. Although they looked like jellyfish, it is still not certain if they were plants or animals or another form of life. Some of the Ediacaran life forms survived into the Cambrian period, but the absence of fossils of most Ediacaran life forms suggests that they became extinct. It is not known if a catastrophe wiped out most species from this early period, or if new life forms evolved that were more successful at surviving and out-competed the earlier Ediacaran forms.
In 1995, two Chinese geologists, Zhu Shixing and Chen Huineng, reported that they had discovered fossils of leaf-like multicellular life forms that were much, much earlier than any discovered before. These ancient plants lived on the ocean floor, near what is now Jixian, China, 1.7 billion years ago. More than 300 fossils were found in the ancient sedimentary deposit. The fossilized forms had various shapes and ranged in size from one-half to ten millimeters wide and five to several tens of millimeters long.
The early fossil record leaves us with some mysteries. Why did it take nearly 2 billion years for complex life to evolve from simple single-celled life forms? What started the evolution of diverse complex life forms? These questions have kept global systems scientists busy for many decades, and many interesting answers have been proposed. All of them are based on some variation of the theory of biological evolution.
II. Theory of Evolution
There is universal acceptance among biologists that the theory of evolution explains the gradual development from simple life forms to the species we see today. This theory was proposed in 1858 by Charles Darwin and also by Alfred Russel Wallace. These naturalists had independently observed that offspring are rarely just like their parents—slight variations or modifications occur randomly. Some differences may reduce the chances that the organism will survive, but sometimes the modification will enhance the organism’s chances to survive and reproduce, and pass on to the next generation the new characteristic. Organisms, then, that are best suited to their environments are most likely to survive to reproduce. This process is called natural selection.
When populations were separated through migrations or when continents separated and drifted apart, the divided populations evolved differently because certain individual differences would have been advantageous in one environment, but not in another. For example, in colder environments the development of fur or body fat would help the organism to survive, but in warmer environments such individual differences may be neutral or even harmful.
When Darwin visited the Galapagos Islands, he noted that certain birds called finches had differently shaped beaks on the different islands. He observed that each island differed in its physical features and plant life. Darwin studied these birds and reasoned that they had evolved from one kind of finch and were now 13 species of finches. Over thousands of years, Darwin reasoned, some birds would have been born with slightly different beaks due to random modifications. The different physical conditions on the different islands would have favored birds with one kind of beak over another. The birds that survived passed on their modified beaks to their offspring, and eventually, a new specie evolved.
Both Darwin and Wallace drew the same conclusion: if the isolation of populations continues long enough—say, several hundred thousand years—the differences between populations would become so great that individuals from the two populations would not be able to mate and reproduce. When they can no longer mate, the two populations become different species.
There are hundreds of studies supporting the theory of evolution, and the framework of this theory is the basis for just about all scientific work on the change of biological organisms through time. The theory of evolution provides a satisfactory explanation for how life evolved since the Cambrian period to the present day. However, many biologists believe that the theory of evolution alone is not sufficient to explain why life evolved so slowly for 3 billion years, and then so rapidly 670 to 570 million years ago. What other factors might have been at work?
III. Why Did the Rate of Evolution Change?
There is no agreement on any one theory explaining why life evolved so slowly in the first 3 billion years of Earth history, and then exploded some 600 million years ago. Let’s consider three theories, all or none of which may be correct.
The development of sex. Simple one-celled organisms, such as bacteria, amoebas, and yeast, reproduce by budding or splitting off part of the cell. This form of reproduction is called asexual, since there is only one set of genetic material. Asexual reproduction results in offspring that are exactly like the parent, except for occasional random modifications. About one billion years ago, some life forms developed two kinds of sex cells, the male gamete and the female gamete. An organism would have either the male or the female gamete, which was one-half the genetic material needed to produce a new organism. When male and female gametes came together, a new organism, which inherited characteristics from each parent, could develop. This process, called sexual reproduction, allowed for much more variety than asexual reproduction. More variations would have led to the evolution of a much wider diversity of life.
Oxygen in the atmosphere. As described in the previous chapter, the first life forms evolved in an atmosphere with little oxygen. Early single-celled life forms gave off oxygen as a waste gas. Eventually, the oxygen accumulated until it made up about 21% of the atmosphere. Oxygen made possible far more efficient use of natural resources by living organisms. It is possible that oxygen gave life the “edge” it needed to diversify.
A change in the climate. Between about 950 and 650 million years ago, the global climate was cool, and large areas of land were covered with glaciers. By 670 million years ago warming was well under way, creating large tropical areas where evolving life forms might have greater opportunities to survive and reproduce.
Single-celled life developed a few hundred million years after Earth cooled and the first continents and oceans formed. As far as we can tell from fossil evidence, these simple life forms were alone in Earth’s oceans for 1.8 billion years before the first multicellular life forms took shape.
It was nearly 3 billion years after Earth formed that a wide diversity of multicellular life forms began to evolve.
Until the 20th century it was believed that the first complex animals evolved during the Cambrian period. The discovery of fossils from soft-bodied animals in the Ediacaran Hills pushed back the time when complex life evolved to a period lasting from about 650 to 540 million years ago, calle the Edicaran(or Vendian) period. The three organisms show on this page (brain coral, radiolaria, and cnidaria) are related to these very early animals.
This chapter describes three recent theories of why life diversified rapidly starting a few hundred million years ago—sexual reproduction, increased oxygen in the atmosphere, and climate change—but no one knows for sure.
Scientists cannot yet explain why life diversified so rapidly in the Cambrian period, more than 500 million years ago. However, there is broad agreement that changes in climate and in Earth itself profoundly influenced the course of evolution. In the next chapter we explore this connection.