History of Evolution on Earth and Origin of Species

HISTORY OF EVOLUTION ON EARTH AND ORIGIN OF SPECIES

There are many ways that the history of evolution on earth can be reconstructed. Much of it includes fossil evidence and carbon dating to indicate when things were present on earth. In addition, modern genetics can be used to estimate when a split occurred between different species. Earlier dates, however, remain mainly speculative as there is little genetic or fossil evidence as to when certain things occurred. What remains is a geological timescale of when things were likely to have occurred on earth as part of the history of evolution and origin of species.

• 3.8 Billion years ago—this is the best guess available for the beginning of life on earth. It is believed that life first began with RNA species rather than DNA species. There was a common ancestor that gave rise to two main groups in life: bacteria and archaea as the first major split in living organisms.

• 3.5 Billion years ago—this is when the oldest fossil records of single-celled organisms first derive from. Shortly after that (about 3.46 billion years ago) the first methanogens (methanogenic archaea organisms) began feeding on methane in the absence of oxygen at this time.

• 3 Billion years ago—this is the first record of viruses, although some scientists believe they were first on earth from the time life first began.

• 2.4 Billion years ago—this is when the waste product of photosynthetic cyanobacteria, oxygen, began to build up in what’s called the “great oxidation event”. This has been challenged with the idea that other bacteria were responsible for this oxidation event. There was a decline in methane-producing bacteria so that oxygen was allowed to build up about this time.

• 2.3 Billion years ago—earth freezes over completely and later melts, leading to more oxygen being released into the atmosphere, further oxygenating the earth.

• 2.15 Billion years ago—there is fossil evidence of photosynthesis from cyanobacteria, believed to be the first photosynthetic organisms. Some believe that this in fact happened earlier than this, during the great oxidation event.

• 2 Billion years ago—this is believed to be when the first eukaryotic organisms developed. It is believed that it happened when a single cell engulfed a smaller cell in an example of endosymbiosis. Engulfed bacteria became mitochondria, providing energy to the cell. Later on, eukaryotic cells engulfed photosynthetic bacteria, which evolved to become chloroplasts in plant cells. This happened on at least three different occasions, resulting in organisms that became green algae and green plants.

• 1.5 Billion years ago—this is roughly when the common ancestors that make up plants, fungi, and animals split into separate cell lines. No one knows which of these three separated themselves out first. Bacteria and archaea had already separated out and were on earth before the first eukaryotes occurred.

• 900 Million years ago—this is believed to be when the first multicellular organisms appeared. It is believed the process was similar to the modern

Choanoflagellates , which form colonies as part of their life cycle. It is otherwise unknown why there was a shift from unicellular to multicellular organisms.

• 800 Million years ago—This is when the early multicellular animals split off from one another. The oldest animals to split off were the sponges. They split off from Eumetazoa (the oldest animal ancestor of all other animals besides sponges). A small group called placozoa broke off about 20 million years after that, which still survive as the oldest type of animal besides sponges.

• 770 Million years ago—there was another ice age, and the earth was covered in frozen water.

• 730 Million years ago—The comb jellies or ctenophores split off from other multicellular organisms. They relied on water flowing through body cavities to acquire nutrients and oxygen.

• 680 Million years ago—the first ancestor of cnidarians (jellyfish and related animals) break away from the other animals.

• 630 Million years ago—this is the first situation of bilateral symmetry with a caudal and cranial (head and tail) end. The closest surviving organism that has

this feature is called Acoela. The earliest fossil record of a bilateral animal organism dates from this era and was a type of worm.

• 590 Million years ago—the organisms called Bilateria, which are animals with bilateral symmetry, split into deuterostomes and protostomes. Deuterostomes split off to become all the vertebrate animals (plus Ambulacraria, an outlier group); protostomes split off to become all of the arthropods (crabs, shrimp, spiders, insects, etc.), certain worms, and rotifers (microscopic invertebrates).

These are differentiated in many ways but, most specifically, by the way their embryos first develop, which is different between the two types.

• 580 Million years ago—this is when the earliest fossils of cnidarians (sea anemones, jellyfish, and coral) have come from.

• 575 Million years ago—the Ediacarans appear but disappear about 33 million years later. These were macroscopic soft-bodied organisms that had fern-like fronds. The period of time they existed in was called the Ediacaran period.

• 570 Million years ago—this is when the Ambulacraria break off from deuterostomes, leading to starfish and their relatives as well as a couple of wormlike families (that aren’t actually worms as worms are not deuterostomes). The sea lily is thought to be the “missing link” between vertebrates and invertebrates and split off at this time. This means that vertebrates, like humans, are closer to starfish than they are to crabs and shellfish.

• 565 Million years ago—this is when the first animal trail fossils were discovered to have come from and when animals are first believed to be moving under their own power.

• 540 Million years ago—the first chordates (animals with a backbone) break off as well as the sea squirts, which have larvae that look like tadpoles that ultimately become bottom-dwelling filter feeders.

• 535 Million years ago—this was when the Cambrian explosion was, with many different new types of fossilized bodies being seen. Some say that this is an

artificial “explosion” because there just weren’t many fossils found prior to that era in evolution.

• 530 Million years ago—this is when the first true vertebrate appears. A vertebrate is an animal with a backbone. It looked much like a hagfish or lamprey and evolved from a jawless fish. Trilobites were first seen at this time and lasted about 200 million years. Figure 32 shows what a trilobite looked like:

• 520 Million years ago—this is when another possible “first vertebrate” appeared, called the Conodonts. These looked much like eels.

• 500 Million years ago—this was when the first land animals existed. The first to do this were called euthycarcinoids, which are the missing link between insects and crustaceans. The oldest known ancestor of cephalopods existed at this time— the precursor of the squid.

• 489 Million years ago—this was a great period of diversity called the Great

Ordovician Biodiversification Event. Many new varieties of plants and animals started to appear at this time.

• 465 Million years ago—this was when plants first began to occur on land.

• 460 Million years ago—this was the major split between cartilaginous fish and bony fish. Cartilaginous fish include the sharks, skates, and rays, and are different from the bony fish, like tuna and other fish.

• 440 Million years ago—this is a split that occurred between two types of bony fish into the lobe-finned fish and the ray-finned fish. Lobe-finned fish became reptiles, birds, amphibians, and mammals, while the ray-finned fish became the fish we know today.

• 425 Million years ago—this is when the coelacanth splits off from lobe-finned fish and is what the modern-day coelacanth looks like today. These species haven’t changed for millions of years. • 417 Million years ago—this is when the lungfish split off from the other lobefinned fish, which are fish but have air-sacs that act as sophisticated lungs along with their gills. They can breathe in or out of water.

• 400 Million years ago—this is when the first woody stems develop as well as the oldest known insect.

• 397 Million years ago—Tetrapods develop at this time, which are the first four-legged animals. These give rise to all land and air animals.

• 385 Million years ago—this is when the oldest fossilized tree first came from, although there had been plants on land for a period of time.

• 375 Million years ago—this is when there was the Tiktaalik, which was an intermediary species between fish and four-legged animals. Fins of this animal develops into limbs.

• 340 Million years ago—the first real tetrapod split occurred with amphibians branching off from the Tetrapods.

• 310 Million years ago—The sauropsids and the synapsids break off from the tetrapods to make the dinosaurs, birds, and reptiles (among the sauropsids) and the synapsids (which become reptiles that had jaws, branching off to make mammals).

• 320 to 250 Million years ago—this is when the Dimetrodons (a type of synapsid) were dominant on land. While these synapsids look much like dinosaurs, they are not of the same classification and are not dinosaurs.

• 275 to 100 Million years ago—this is when the therapsids evolve alongside the pelycosaurs, with some of them, called cynodonts, having dog-like teeth becoming eventually the first mammals.

• 250 Million years ago—this was the end of the Permian period and was a time of mass extinction, when trilobites were killed off. The sauropsids become dominant over the synapsids in the age of the dinosaurs. Mammals still existed but they were small and relatively nocturnal. The modern-day octopus developed at this time as well as the large marine reptiles in the oceans of the world.

• 210 Million years ago—this is when the first bird footprints came from, suggesting that some early dinosaurs were developing into birds during this time.

• 200 Million years ago—this is the end of the Triassic period, when there is another mass extinction. Dinosaurs dominate the earth after this time. The first warm-blooded animals evolved by this time.

• 180 Million years ago—there is the first split in the mammal population to have a group of mammals that lay eggs instead of bearing young. These monotremes gave rise to the duck-billed platypus (which lays eggs as a monotreme).

• 168 Million years ago—there is fossil evidence of a flightless, half-feathered dinosaur in China, which may be the first step toward the dinosaur evolution into birds. At 150 million years ago, the Archaeopteryx lived in Europe as the first known bird there.

• 140 Million years ago—there was a split between placental mammals and marsupials like kangaroos. They did not originate in Australia but came from

South-East Asia to North America, South America, and finally Antarctica and

Australia.

• 130 Million years ago—the first flowering plants came to be on earth.

• 105 to 85 Million years ago—there was a major split among placental animals into four different groups: 1) Laurasiatherians (hoofed mammals, whales, bats, and dogs), 2) Euarchontoglirians (primates, rodents), 3) Xenarthra (anteaters and armadillos), and 4) Afrotheres (Elephants and aardvarks).

• 100 Million years ago—this is when dinosaurs reached their largest size on earth, including the giant sauropod, the Argentinosaurus.

• 93 Million years ago—there is a starvation of oxygen within the oceans during this time, possibly because of a volcanic eruption, wiping out more than a fourth of all marine invertebrates.

• 75 Million years ago—this is when the ancestors of modern primates split off from rodents and rabbits. Rodents ultimately go on to represent 40 percent of modern mammal species.

• 70 Million years ago—this is when the first grasses evolve but they do not become prominent for several million years.

• 65 Million years ago—this is when the infamous Cretaceous-Tertiary (K/T) extinction occurred, wiping out the dinosaurs and related reptiles. This goes on to pave the way for mammals to dominate the earth.

• 63 Million years ago—this is when primates split into dry-nosed primates and wet-nosed primates. The wet-nosed primates evolved to become lemurs, while the dry-nosed primates became monkeys, apes, and humans.

• 58 Million years ago—the tarsier splits from the rest of the dry-nosed primates. It had very large eyes in order to see better at night.

• 55 million years ago—there is a sudden rise in greenhouse gases that raise the temperature of the earth, wiping out many species in the deeper oceans; this was called the Paleocene/Eocene extinction.

• 50 Million years ago—this is when the Artiodactyls, a cross between a wolf and a tapir, begin evolving into whales.

• 47 Million years ago—this is when early whales called protocetids live in shallow seas and go to land in order to give birth.

• 40 Million years ago—This is when higher primates diverge from the rest of the primates to colonize South America.

• 25 Million years ago—this is when apes split off from Old World monkeys.

Around 18 million years ago, gibbons split off; around 14 million years, orangutans split off from other great apes to live in Asia; about 7 million years ago, gorillas split off from the other great apes.

• 6 Million years ago—the first humans diverge from their closest relatives: the chimpanzees and the bonobos. It is shortly after this that man becomes a twolegged walker.