“ALL CREATURES GREAT AND SMALL:
THE CAMBRIAN EXPLOSION OF ANIMAL LIFE AND ITS AFTERSHOCKS”
1 billion to 100 million years ago
| “The March of Life.” The progression of life from a “primordial soup” to more and more complex life forms over time.
Credit: Illustration by John Sibbick, courtesy of John Sibbick Illustration. This illustration is used with permission. All rights reserved.
How did the design of animal body plans emerge during the course of evolution? During Time Scale Two, the first complex animal forms emerge from worm-like ancestors. After a number of extreme ice ages, the last “Snowball Earth” phase occurred about 635 million years ago. The subsequent warm period may have created favorable conditions in the sea leading to the “Cambrian Explosion,” which occurred about 540 million years ago. All of the modern animal groups emerge at this time, including annelids (worms), jellyfish and anemones, sponges, brachiopods (“lamp shells”), bryozoans (“moss animals” or “sea mats”), mollusks (snails, clams, squid, octopi), arthropods (crabs, spiders, insects), and chordates (our ancestors), and the major developments leading to the emergence of the dinosaurs and early mammals are documented in the fossil record.
The major stages of animal evolution that emerge during this period include chordates, vertebrates, jawless fishes, jawed fishes, lungfish, amphibians, reptiles, mammal-like reptiles, and finally mammals. The greatest extinction event in the history of life, the Permian Extinction, occurred around 250 million years ago, with over 90 percent of the earth’s species going extinct.
During this Time Scale the first land plants also populate the continents, and the African plate separated from the North American plate (creating the Atlantic Ocean). The earth was populated with the major groups (phyla) of animals we know today, although the fossil record preserves earlier, ancestral forms for these animal groups.
“MOVERS AND SHAKERS”
THE FIRST DEFINITE MULTICELLULAR ANIMALS
600 million years ago
For the first three billion years, the fossil record mainly documents stromatolites, rocks built up by the slow growth of microscopic single-celled bacterial colonies. Quite interesting, but very, very monotonous. With the end of the Second Snowball Earth about 600 million years ago, and the general warming of the earth’s climate that followed, things began to get really, really interesting…
THE CAMBRIAN EXPLOSION
543 million years ago
545 million years ago, there was a dramatic change in the diversity of animal life. Starting from simple worm-like forms, an astonishing range of more complex animals emerged, including all of the major groups (or ‘phyla’) of modern animals. This radiation occurred after the Precambrian “Snowball Earth” ice age, when the earth warmed, sea levels rose, and the supercontinent of Gondwanaland began to break up, creating many new marine habitats along shallow continental shelves. The earliest chordates, or animals with a notochord (a flexible rod that runs the length of the body and protects the nerve cord), include the two-inch fossil Pikaia, which could be a direct human ancestor.
“IN SEARCH OF THE LOST CHORDATE”
FINDING THE ANCESTOR OF VERTEBRATES (AND OURSELVES)
530 million years ago
The Cambrian explosion created all the major phyla of animals that we have today. This includes the phyla Chordata (chordates). Primitive forms are creatures with evidence of a notochord, a structural rod of cartilage and neural tissue running down the long axis of the body. This notochord was the early precursor to the spine in vertebrates (animals with a true backbone, like fish), and modern examples of simple chordates include sea squirts and lancelets. Two Cambrian fossil localities are of key importance here: Chengjiang in Yunnan Province, southern China, and the Burgess Shales in the British Columbia Rockies of Canada. Both of these localities have produced fossils classified as chordates. The Chinese forms include the forms Myllokunmingia and Haikouichthys (it has even been suggested these could be primitive vertebrate jawless fish). The Canadian form (somewhat later in the Cambrian period) is Pikaia, which especially resembles the modern lancelet.
“GET A BACKBONE”
THE SPREAD OF THE VERTEBRATES
525 million years ago
Although vertebrates make up only about 5% of animals alive today, most of the animals we are familiar with today – that we see on a regular basis both domestic and wild – are vertebrates. We are vertebrates, along with fish, birds, amphibians, reptiles, and all of the other mammals. All vertebrates are based on the basic chordate body plan that emerged slightly earlier, but have the important addition of a spinal column or backbone along with a brain case (such a brain case is only found in some early chordates, the “craniates”). Vertebrates have a definite “front end” – with sensory organs such as eyes and a concentration of nervous tissue, burgeoning early brains in the early forms. Early vertebrates appear fairly early in the Cambrian, 525 million years ago, about 5 million years after the Cambrian ‘explosion’ began. These did not look like modern vertebrates, but show the early development of this type of body plan – by this time early animals had managed to get a backbone and get a head. Although not technically fish, the early jawless fishes develop out of these early vertebrates during the next 100 million years. Plants also started getting more complex, with green algae apparently developing into forms that finally colonized the land, included types of early liverworts and fungi.
“GET OUT OF THE POOL”
360 million years ago
We are land-living animals with four limbs – so are most of the animals we know and love. But go back in time 400 million years or so, you would not see any such animals. Life was primarily in the sea, which teemed with invertebrates like trilobites (extinct relatives of spiders and crabs) and brachiopods as well as early fish. On land there were some arthropods (early spiders and insects, including a scorpion the size of a man!) starting a life on a landscape studded with early land plants. But starting 360 million years ago this began to change, when creatures called ‘tetrapods’ (meaning ‘four-legged) began to move out of the seas. Descendants of early fish – in fact they looked like fish with thick, muscular fins – they were already adapted to water. But living in shallow water swamps, they were also beginning to adapt to land. These tetrapods were the ancestors of amphibians, whose very name means “double life,” as most of them spend some on the land (as adults) but tend to lay their eggs in the water. Amphibians today include frogs, toads, newts, and salamanders.
THE FIRST REPTILES
315 million years ago
Amphibians are very tied to water – to lay their eggs, and often to live, at least for part of their life. Reptiles made a break from this by 315 million years ago, due to the evolution of a new kind of egg – one with an outer membrane that would protect it from drying out. This might sound trivial, but it was really, really important. Reptiles are cold-blooded, just like amphibians. But this new egg would let the evolving reptiles, living on land alongside early amphibians more than three hundred million years ago, to loosen their ties to swamps, lakes and seas, and also move into other parts of the landscape (and some also returned to the water!). This transition happened during the time when the great coal deposits of the entire world were laid – a period called the “Carboniferous,” when great swamps of impressive plants and a humid, tropical climate prevailed over most of the planet. Now reptiles inhabit virtually every continent (except Anarctica) and live in habitats ranging from tropical forests, lakes and swamps, to the driest of deserts.
“THE GREAT DYING”
THE PERMIAN EXTINCTION
250 million years ago
Catastrophic environmental changes due to one of the largest volcanic episodes in our planet’s history 250 million years ago led to the extinction of over 90 percent of the earth’s life forms. This catastrophe also set the stage for the emergence of the dinosaurs and the earliest mammals, including our ancestors.
EARLY MAMMALS AND DINOSAURS
220-200 million years ago
Most people don’t realize it, but the class of animals that we belong to, the mammals, evolved from mammal-like reptiles at the same time as the emergence of early dinosaurs, around 200 million years ago. Mammals and dinosaurs therefore co-existed for some 135 million years before dinosaurs went extinct. Although small and insignificant at first (and probably nocturnal, or moving and feeding at night), they would later become a major player in the History of Life. These early mammals established the foundation of what would become the human condition.
“UP, UP, AND AWAY”
THE EARLIEST BIRDS
145 million years ago
Bird-watchers are fascinated by the diversity and beauty of our fine-feathered friends, and humans have envied the freedom and gracefulness of their flight for thousands of years. Birds are a warm-blooded class of vertebrates characterized by feathers, wings, and, for most, the ability to fly. How far back can we trace such creatures in the fossil record? Beginning around 152 to 168 million years ago some smaller reptiles began to develop feathers, probably first to help regulate body temperature and later selected for flight. The earliest known feathers found on reptiles in China may have developed to provide warmth, rather than flight. Birds are believed to have evolved from a form of small, bipedal theropod dinosaur, and birds are considered by most paleontologists to the one group that survived the dinosaur extinction 65 million years ago.
THE FIRST FLOWERING PLANTS
125 million years ago
When you think of flowers, you probably think of bouquets, corsages, and perfumes, but you may not realize that flowering plants (called angiosperms) are the dominant form on the earth today, with over 300,000 species known. Before the first flowers, the majority of plants were ferns (including giant, tree-like forms), bushes such as “horsetails”, conifers (including pines, firs, and spruces), and ginkgos. Most of our oil and coal deposits were made from the decomposing matter from these early plants. Starting about 125 million years ago, plants began to develop flowers, essentially sexual organs for reproduction.