Marine Diversity I Sept 26, 2002

First exam is next Tuesday, Oct. 1. Sample exam is on web page

Marine diversity I

The five kingdoms

Precambrian time

Key vocabulary

Greatest hits of the Precambrian

Lessons of the Precambrian

The Five Kingdoms

First I want to give you some evolutionary context to the following lectures on the diversity of marine and terrestrial life, and to the labs on the major fossil groups. I'll do this by looking at where the diversity of life in a broad evolutionary sense.
Biologists now recognize at least five kingdoms: five major categories of life. They are illustrated on the handout, which is taken from the book by Margulis and Schwartz.

Let's have a look, and I will highlight the major fossil groups:

1. Monera - bacteria, including cyanobacteria (formerly known as blue-green algae). Mostly single-celled. In rock record. What few pC fossils there are, are members of this kingdom and most are cyanobacteria.

2. Protoctista - more commonly called Protista. Microscopic, mostly single-celled. Most algae, "protozoans" Important fossil groups include Foraminifera, Dinoflagellata (acritarchs), Haptophyta (coccoliths), Chlorophyta (green algae) Actinopoda (radiolaria) Bacillariophyta (diatoms) Rhodophyta (red algae).

3. Fungi. mushrooms, molds, yeasts, lichen. Very rare in the fossil record. Multicellular. No important fossil groups.

4. Animalia - invertebrates (not a formal taxon) and vertebrates. Multi-cellular. Many in the record but many are not. Important fossil groups include: Porifera, Cnidaria, Brachiopoda, Ectoprocta, mollusca, arthropoda, echinodermata, Chordata

5. Plantae - plants, from mosses to ferns, conifers, to flowering plants. Spotty but important fossil record. Coniferophyta, Cycadophyta, Angiospermophyta.

Consider a mushroom and pepperoni pizza... representatives of all five kingdoms

Lessons of this illustration:

1. Great diversity of life forms at the microscopic level: monera and protista.

2. Because most life forms don't have hard parts, most are not represented in the rock record. The fossil record of evolution is only a sample of the evolution that went on.

Precambrian time

Talking about Precambrian paleobiology requires an even greater perspective on geologic time than just the past 540 or so million years.. While most of the fossil record is concentrated in the last 540 million years of Earth history, the story of life on earth begins much much earlier.

Here's a full time scale, back to about 4.5 billion years ago, the estimated time of the formation of the earth.

Cenozoic

Phanerozoic Mesozoic

Paleozoic

Cambrian

Vendian

Proterozoic

Archaean

The date of 4.5 billion years for the approximate age of the earth is based on the ages of meteorites and moon rocks.

The oldest rocks on earth are about 4 billion years old, from northern Canada and the oldest sedimentary rocks are about 3.8 billion years old, from Greenland.

The point here is that we don't have much direct evidence left on earth of what the earth's rocks and environments were during the first half billion years or so of its history. This is because the earth's surface is active, with uplift, metamorphosis and erosion tending to alter or destroy older rocks.

Here's the vocabulary (overhead)

heterotrophic - organisms that obtain metabolic energy by breaking down molecules absorbed from the environment. We are heterotrophs, as are clams, dinosaurs, sponges, and most animals and many forms of single-celled life.

autotrophic - organisms that absorb external energy and use it to build up food internally - photosynthesis, for example, allows organisms to be autotrophic. Plants are autotrophs, as are many forms of single-celled life.

anerobic - needing to live in the absence of oxygen. Many bacteria are anerobic.

aerobic - needing oxygen in order to live. Most organisms are aerobic.

procaryotic - is a cell type that does not have a distinct nucleus. Asexual reproduction only. Bacteria are procaryotes.

eucaryotic - is a cell type that does have a distinct nucleus as well as other distinct bodies (mitochondria, chloroplasts, etc.) within the cell. Asexual and sexual reproduction. All eucaryotes are aerobic. All multicelluar forms of life are composed of eucaryotic cells. Animals, plant, fungi, and many single-celled forms are eucaryotic.

Review greatest hits of Precambrian organisms (handout):

0.6 billion years - oldest metazoans (multicellular animals)

1.6 billion years - oldest acritarchs (eucaryotic cells)

2.5 billion years - abundant stromatolites; oxygenation of atmosphere (Archean/Proterozoic boundary)

3.5 billion years - first evidence of life (stromatolites, cyanobacteria?)

3.8 billion years - oldest sedimentary rocks

4.5 billion years - age of the earth

Where does this get us?

In most scenarios the earliest cells are heterotrophs deriving their energy from external sources - "eating" as it were, the organic compounds around them and deriving energy from their chemical bonds. This is a much simpler biochemical system than photosynthesis. Remember that such heterotrophs would need to be anerobic -living in the absence of oxygen.

Indeed, the physical evidence from the rocks (as well as models of planetary evolution) suggest that the earth's early atmosphere did not have oxygen.

Such organisms exist today among the bacteria, and the first life was probably quite like many bacteria. Bacterial cells are said to be procaryotic, meaning that the cell does not have a well-defined nucelus.

- the first life was probably heterotrophic, anerobic (no O₂ in the atmosphere, remember), and procaryotic.

The first fossil evidence for life comes from some rocks in Australia called the Warrawoona Series, Ajax Chert dated at about 3,550 million years old.

-the evidence is in the form of stromatolites: sedimentary structures consisting of low mounds or domes of finely laminated sediment

-stromatolites form today is some isolated marine and fresh water environments. They are made by mats of cyanobacteria - sometimes called blue-green algae. Very simple, but nevertheless, photosynthetic (thus autotrophic) bacteria.

-stromatolites are probably the most common evidence of Precambrian life.

-The late Achean and early Proterozoic saw a big increase in the abundance of stromatolites, suggesting widespread colonies of blue green algae. The world was probably a pretty slimy place.

The stromatolites did more than make things slimy - they gave off oxygen as a byproduct of their photosynthesis. In considering the effect that life has on the physical environment of the earth, this is the most profound effect of all. The fact that the earth has free oxygen in its atmosphere is thanks to photosynthetic activities - dating back to the slimy Archean cyanobacteria.

At first, all the oxygen given off by the cyanobacteria went into oxidizing minerals - especially iron minerals:

(overhead)

-Banded Iron Formations are sedimentary rocks made up of alternations of iron ore and chert, sometimes very, very finely laminated. Banded Iron Formations are dominantly found in rocks older than 1,800 million years. They make up thousands of meters of rock and are not forming today.

-the idea is that as the cyanobacterial mats gave off oxygen, it oxidized dissolved iron in the water, which then precipitated out to form the sedimentary deposits.

-Oxygen then began to build up in the atmosphere after most of the available iron was oxidized. Free oxygen was then built up in the oceans and atmosphere.

-another byproduct of oxygen in the atmosphere is the formation of the ozone (O-3) layer, shielding the surface of the earth from excess UV and cosmic radiation.

-Recall that eucaryotic cells are exclusively aerobic - they need oxygen in order to live.

-So these more complex forms of life had to wait until enough oxygen had build up in the atmosphere.

Precambrian time

Precambrian - the era of microscopic life