Chapter 25: History of Life

Types of Fossils:

1. Intact Fossil — fossil that forms when decomposition doesn’t happen and the organism remains preserved and in tact. Extremely rare, and very useful when found.

2. Compression Fossil — fossil that forms when sediments accumulate on top of the organism and cement the organism into rocks. Super common, usually soft-bodied organisms like plants and invertebrates.

Downsides to fossil evidence:

• They’re super rare

• They’re only tiny blips of the ginormous history of life on Earth

• Habitat bias: organisms living underground are way more likely to fossilize

• Taxonomic and Tissue Bias: organisms with hard parts are most likely to fossilize, some tissues fossilize way easier than others

• Temporal bias: recent fossils are much more common than ancient fossils

• Abundance bias: the fossil record is much better for common species. Example: in millions of years, house cat fossils will probably be a lot more likely to be found than tiger fossils due to house cat’s abundance.

Eras of Life:

Precambrian Era: the interval between the formation of Earth around 4.6 billion years ago and the appearance of most modern animal groups 541 million years ago

• Origin of prokaryotic life

• Cells start performing photosynthesis, they fill the atmosphere & ocean with oxygen

• Cyanobacteria heavily produces oxygen

• First eukaryotic fossils, some of them are photosynthetic

• First red algae appears, first evidence of sexual structures

• Sponges first appear

Phanerozoic Eon

The interval between around 541 million years ago to the present. There are 3 major eras:

1. Paleozoic era: begins with the appearance of most major animal lineages and ends with the obliteration of almost all multicellular life. Initial diversification of animals, land plants, and fungi. Appearance of land animals.

2. Mesozoic era: begins with the extinct of the paleozoic era and ends with the extinction of the dinosaurs (except birds). Gymnosperms were dominant on land; dinosaurs were dominant vertebrates. This is really the era of the dinosaur.

3. Cenozoic era: flowering plants dominate land, birds and mammals are dominant vertebrates. This is the current era of Earth.

Adaptive Radiation

Adaptive radiation: when a single lineage rapidly produces many descendant species with a wide range of adaptive forms. Opposite of extinctions.

Why do adaptive radiations occur?

1. Ecological opportunity: availability of more resources or new types of resources

2. Morphological, Physiological, or Behavioral Innovation: evolution of a new key trait. Example: flowering plants evolved like crazy due to their hip new trait: flowers

The Cambrian Explosion!

Possibly the most famous case of an adaptive radiation. Early macroscopic soft bodied animals diversified to include hard bodied, multi-segmented animals with brains!

Why did this happen??

1. Oxygen increased in the atmosphere and ocean (ecological opportunity)

2. The development of hox genes (genes that are super important for animal development) (physiological innovation)