Chapter 25 (3)

The study of fossils has helped geologists to establish a __ of Earth's history, which is divided into three __. The first two __- the __ and the __- together lasted aprox. 4 bil years. The __, roughly the last half billion years, encompasses most of the time that animals have existed on Earth. It is divided into three eras: the __, __ and __. Each era represents a distinct age in the history of Earth and its life.

For example, the __ is sometimes called the "__" becuase of its abundance of reptilian fossils, including those of dinosausr. THe boundaries between the eras correspond to major extinction events seen in the fossil record, when many forms of life disappeared and were replaced by forms that evolved from the survivors.

The __ provides a sweeping overview of the history of life over __ time.
- THe earliest evidence of life, dating from 3.5 bil years ago, comes from fossilized __, which are layered rocks that form when certain prok bind thin films of sediment togeter. Present-day __ are found in a few warm, shallow salty bays. If microbial communities complex enough to form __ existed 3.5 bil years ago, it is a resonable hypothesis that single celled organisms origninated much earlier, perhaps as early as 3.9 bil years ago.

Most atmospheric oO2 gas is of biological origin, produced during the water splitting step of _-. When oxygenic __ first evolved the free O2 it produced prob dissolved int he surrounding water until it reached a high enough concentration ot react with dissolved iron. Thus would have caused the iron to precipitate as __, which accumulated as sediments.

These sediments were compressed into banded __, red layess of rock containing iron oxid that are a source of iron ore today. Ocne all of the dissolved iron had precipitated, additional O2 dissolved int eh water until teh seas and lakes became saturated with O2. After this occured, the O2 finally began to __ of the water and enter the atmosphere. THis change left its mark int he rusting of iron-rich terrestrial rocks, a prcess beginning about 2.7 bil years ago. This chronology implies that bacteria similar to today's _ (O-releasing photosyn. bacteria) originated well before 2.7 bil years ago.

The amount of atmospheric O2 increased gradually from about 2.7-2.2 bil years ago, but then shot up rapidly to more than __ of its present level. THis _ had an enormous impact on life. Inc ertain of its chemical forms, oxygen attacks chemical bonds and can inhibit __ and damage cells. As a result the rising concentration of atmosspheric O2 prob doomed many prok groups. Some species survived in habitats that mremained __, where we see their descendants. Among other survivors, diverse adaptations to teh changing atmosphere evolved, including __ which uses O2 int he prcess of harvesting teh energy stored in organic moleucles.

The early, gradual rise in atmospheric O2 levels was prob brought about by ancient __. A few hundred mil years later, the rise in O2 accelerated. What aused this aceleration?
One hypothesis is that this rise followed the evolution of euk. cells containing __.

Besides organelles, unlike prok cells, euk cells have a __, a feature that enables euk cells to change their shape and tehreby surround and engulf other cells.

How did these euk features evlove from prok cells?
A rang of evidence supports a model called __, which posits that mitochondria and __ (a general term for chloroplasts and related organelles) were formerly small prok that began living within larger cells. The term __ refers to a cell that lives within another cell, called the __. The prok ancestors of mitochondria and plastids prob gained entry to the host cell as undigested prey or internal parasites. Though such a sprocess may seem unlikely, scientists have directly observed cases in which __ that began as prey or parasites came to have a mutually beneficial relationship with the host in as little as five years.

by whatever means the relationships began, we can hypothesize how the __ could have become mutually beneficial. A heterotrophic host could use nutrients released from photosynthetic __. And in a world that was becoming increasingly __, a host that was itself an __ would have benefited from __ that turned the oxygen to advantage. Overt ime, the hose and __ would have become a single organism, its parts inseparable. Although all euk have mitochondria or remnants of these organelles, they do not all have plastids. Thus, the model of __ supposes that mitochondria evolved before plastids through a sequence of endosymbiotic events.

A great deal of evidence supports the __ origin of mito and plastids. The inner membranes of both have enzymes and transport systems that are homologous to those found in the plasma membranes of living prok. Mitochondria and plastids replciate by a splitting process that is similar to that of certain prok. In addition, each of these organelles contains a single, circular DNA molecule that, like the chromosomes of bacteria, is not associated with __ or large amounts of other proteins. As might be expected of organelles descended from free-living organisms, mito and plastids also have the cellular machinery (including __ ) needed to transcribe and translate their DNA into proteins. Finally, in terms of siez, nucleotide sequence and sensitivity to certain antibiotics, teh ribosomes of mitochondria and plastids are more similar to _- ribosomes than they are to the cytoplasmic ribosomes of __ cells.

The appearance of structurally complex euk cells sparked the evolution of greater morphological diversity than was possible for the simpler prok cells. After the first euk appeared, a great range of unicellular forms evolved, giving rise to the diversity of single-celled euk that continue to flourish today. Another wave of __ also occurred: Some single-celled euk gave rise to multicellular forms, whose descendants include a variety of algae, plants, fungi, and animals.

Based on comparisons of __, researchers suggest that the common acncestors of multicellular euk lived 1.5 billion years ago. This result is in rough agreement with the __; the oldest known fossils of multicellular euk. are of relatively small algae that lvived about 1.2 bil years ago. Larger and more diverse multicelllular eukaryotes do not appear in teh __ until about 565 mil years ago. These fossils, referred to as teh __, were of soft-bodied organisms- some over 1 m long- that lived from 565 to 535 mil years ago.

Why were the multicellular euk limited in size and diversity until the late Proterozoic?
o __evidence indicates that a series of severe ice ages occurred from 750-580 mil years ago. At various times during this period, glaciers covered all of the planet's landmasses, and the seas were largely iced over.

The __ suggests that most life would have been confined to areas near deep-sea vents and hot springs or to equatorial regions of the ocean that lacked ice cover. The __ of the first major diversification of multicellular euk (beginning about 565 mil years ago) corresponds roughly to the time when snowball earth thawed. As that _ came to a clsoe about 30 mil years later, the stage was set for another, even more spectacular burst of evolutionary change.

many phyla of living animals appeared suddenly in foosils formed early int eh __ period (535-525 mil years ago), a phenomenon referred to as the __. Fossils of at leasat three living animal phyla- __ (sea anemones and their relatives), __ (sponges), and __ (molluscs)- appear ine ven older rocks dating from teh late __.

Prior to the __, all large animals were soft-bodied; and fossils show little predation. Instead the animals were herbivores, filter feeders, or scavengers- not hunters. The __ changed that in a short pd of time (10 mil), with predators of over 1 m in length emerging with claws and other 'capturing prey' features; simultaneously, new defensive __, such as sharp spines and heavy body armor, appeared in the prey. The safe, slow-moving wolrd that characterized larger forms of life before th _- was gone forever.

Although the __ had a large impact on life on Earth, it's possibel that many animal phyla originated longe before that time. Some DNA analyses suggest that most animal phyla originated and began to diverge from one another as early as 700 mil to 1 bil years ago. Even if these estimates are not correct, recent fossil discoveries in China suggest that animals similar to members of living animal phla were present 10s of mils of years before the __. The discoveries include 575 mil year old fossils of beautfully preserved specimens interpreted by some scientists as animal __ and by others as members of extinct groups closely related to animals.

Overall, as noted by Simon Conway Morris, "the __ had a long fuse"- at least 40 mil years long, based on CHinese fossils. THe fuse may have been 100s of millions of years long if some animal phla originated as far back as some DNA based estimates suggest.

The __ was anothe rmilestone int eh history of life. There is fossil evidence that __ and other photosynthetic prok. coated damp terrestria surfaces wel over a bil years ago.However, larger forms of life did not begin to lcolonize land until about 500 mil years ago. This gradual evolutionary venture out of aquatic environmensts was associated with adaptations that made it possible to reproduce on land and that helped prevent dehydrateion.

Plants colonized land int eh company of __. Such mutually beneficial associations of plants and fungi are evident in some of the oldest fossilized roots, dating this relationship back to the early spread of life onto land.

Although many animal groups are now represented in terrestrial environments, the most widespread and diverse land animals ae __ (insects and spiders) and __. THe earliest __ found int he fossil record lived about 365 mill years ago and appear to have evolved from a group of lobe-finned fishes. __ include humans, which diverged from primates (6-7 mil years ago) and our species origninated only about 195, 000 years ago.