Additional phylogenetic approaches has allowed us to propose a model (figure 4 at right) on the origin of photosynthesis which states that photosynthesis is a process that was derived from cytochromes that predate components of the photosystem (Xiong and Bauer, 2002)
More recently, we have undertaken a more ambitious project centered on the origin of microbial life. For this analysis, we have teamed up with Dr. Lisa Pratt, a geo-microbiologist at IU that is undertaking geochemical analysis of a region of Western Oregon known as Warner Valley . This area has a series of shallow alkaline (pH 8 to 10) lakes and ponds that undergo frequent seasonal periods of drying (summer and fall) coupled with periods of hydration (winter and spring). These frequent seasonal fluctuations lead to significant variations in salinity. This fluctuating environmental condition of hydration is thought to mimic environmental events that occurred during early Earth history and during dehydration of the planet Mars.
The earliest known fossils of most of the modern orders of mammals appear in a brief period during the early Eocene (55.5 - 33.7 million years ago). Both groups of modern hoofed animals, the ("even-toed" taxa such as cows and pigs) and ("odd-toed" taxa, including the horses), became widespread throughout North America and Europe. The evolutionary history of the horses is particularly well understood: Stephen Jay Gould (1983) provides an excellent discussion of it in his book "Hens' teeth and horses' toes".
Thousands of trilobite species evolved during the Paleozoic Era, the last of them dying out around 250 million years ago. Paleontologists have traced some of their more exuberant decorations to the evolution of more rapacious trilobite predators. This process probably started in the Ordovician Period, which began around 485 million years ago. Some trilobites evolved spikes to puncture would-be diners while others burrowed under the sea floor and left just the tips of their stalk eyes at the surface. Recent research has deduced the evolution of another defensive feature. Trilobites could roll up like a modern-day pill bug or armadillo. A 2013 study found that this useful posture evolved early on in trilobite history, though enrollment tricks got more sophisticated over time. Early trilobites had to maintain their rolled-up posture through muscular effort; later trilobites came outfitted with body parts that fit together like locks, frustrating bigger animals trying to pry the bugs open. Other researchers have found similarities between the features of ancient trilobite eyes and the peepers of modern horseshoe crabs — "a 'living fossil,' which probably retained this ancient basal system successfully until today."
New Zealand, by virtue of its isolation and its relatively recent geological development, was not the centre of any novel evolutionary development. However, many of the species that date back to Gondwanaland, or that arrived more recently as migrants, have undergone significant adaptive radiation in their new homeland. Some of the best examples of this can be related to the major ecological changes that accompanied the Pleistocene Ice Ages.
Fossils that have been found in great abundance date back to the Cambrian Period, which began about 545 million years ago. Trilobites first show up in the fossil record over 500 million years ago, and fossil lovers have collected trilobites for centuries. Naturalist Edward Lhwyd published a description of a "flatfish" in 1679. In the Victorian era, a well-polished trilobite was mounted in a gold pin known as the Dudley Locust Brooch, now on display at the Natural History Museum, London. Trilobite fossils vary so much in size that some are best measured in millimeters while others are better measured in feet. Some sport spines, others have eyes on the ends of stalks. Yet these ancient water bugs remain recognizable as trilobites thanks to their shared body plan: horizontal segments spanning three lengthwise sections. No matter how big and fancy some of them got, they shared the same ancestral shape.
So the fossil record is spotty at best, but even with so many pieces missing, it tells us quite a bit about the history of life on Earth, and more discoveries happen all the time.
Understanding life's earliest history on Earth requires understanding what life is, and the definition turns out to be tricky. In general, life reproduces itself and passes along traits, life maintains itself by changing energy and matter, and life has some physical boundary that separates itself from the outside world. But the border between life and non-life is surprisingly fuzzy. Viruses have genetic code and some other trappings of living cells, but must rely on their hosts to reproduce. Prions (the culprits spreading mad cow disease) are just proteins folded in a weird way. Even crystals can reproduce via cloning. None of these things fit the definition of life, but they skulk along the margins. They are not unusual. They have been around a long time, including billions of years ago, when our planet was young.
The fossil record is littered with exceptions, but in general, bones, teeth and exoskeletons are more likely to fossilize than soft tissue. (Your minteral-rich teeth are practically fossils-in-waiting the moment they erupt from your gums.) Trilobite shells, even the ones molted off, preserve reasonably well, as do the animals' calcite-crystal eyes. The vast majority of trilobites never left behind fossils, but they fossilize better than many other animals. Historically, paleontologists have been more likely to notice big fossils than little ones, though this is changing, and big plants or animals may have a better shot at being preserved in the first place.
When the Earth was forming, harsh conditions were probably the norm on our planet, although when those conditions eased is a matter of ongoing study and debate. About 4 billion years ago, the solar system was young and conditions were violent, with giant chunks of rock careening into planets regularly. Our own moon may have been formed by one such impact. In 2014, a team of Czech scientists conducted an experiment to replicate the pressure-cooker conditions created by an asteroid slamming into the Earth. The researchers used a high-powered laser to blast a solution of formamide — a chemical that forms from the reaction of hydrogen cyanide and water, and was likely abundant in the early days of our planet. The experiment produced adenine, guanine, cytosine and uracil: the nucleobases of RNA. The experiment suggested that, rather than impeding the development of life, the violent impacts of our early solar system might have contributed to its formation. On the other hand, some studies indicate that our planet might have cooled relatively early on, and hosted some forms of simple life. Zircon crystals from western Australia, dated at over 4 billion years old, have been found to contain oxygen isotopes suggesting that the crystals formed under cool, moist conditions. A 2015 paper by UCLA geologists focused on a Western Australia zircon crystal containing graphite. The authors argued that the carbon isotopes in that crystal constituted "probable" evidence of the occurrence of photosynthesis on Earth as long as 4.1 billion years ago. Vigorous debate about this finding, though, would be more than probable.
By 125 million years ago the mammals had already become a diverse group of organisms. Some of them would have resembled today's (e.g. platypus and echidna), but early (a group that includes modern kangaroos and possums) were also present. Until recently it was thought that mammals (the group to which most living mammals belong) had a much later evolutionary origin. However, and DNA evidence suggest that the placental mammals are much older, perhaps evolving more than 105 million years ago. Note that the marsupial and placental mammals provide some excellent examples of , where organisms that are not particularly closely related have evolved similar body forms in response to similar environmental pressures.
How could we wind up with genetic material from different domains? It could be that, when life was young, RNA and DNA didn't have much competence in self-replication, and mutations ran rampant. Genetic replication eventually improved. Likewise, if cell membranes became more sophisticated, they would get better at keeping out intruders (including genes). So the lateral gene transfer that was once commonplace, and still occurs in bacteria today, has probably become much rarer in multicellular organisms. In short, not only has life evolved, evolution has evolved.