Page 203 - Introduction to Paleobiology and The Fossil Record
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190 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD
banded iron formations occurred worldwide; kingdoms, Protoctista for single-celled eukary-
these consist of alternating bands of iron-rich otes and Monera for prokaryotes. Four king-
(magnetitic/hematitic) chert and iron-poor doms became five in 1969 when Robert
chert (chalcedony). In the Archaean, iron Whittaker recognized that Fungi (mush-
released from vents in the seafl oor was mobile rooms and molds), classed by chefs as plants,
in the deep ocean and welled up onto the are fundamentally different from all other
continental shelves. This is unlike today, plants.
where oxygen extends to the bottom of the This five-kingdom picture of life was blown
sea and iron is immediately deposited as an out of the water by a series of revolutionary
oxide on the flanks of mid-ocean ridges. The papers by Carl Woese and colleagues from the
banding in banded iron formations may refl ect University of Illinois from 1977 onwards.
seasonal plankton blooms that released a Woese and George Fox had been working on
great deal of oxygen into the surface ocean, molecular phylogenies (see p. 133) of pro-
which combined with upwelling iron ions to karyotes, and they realized that prokaryotes
produce the iron-rich layers. About 1.9 Ga, fell into two fundamental divisions, the
banded iron formations largely disap- domains Archaea (named Archaebacteria by
pear. Continental red bed sediments had fi rst Woese and Fox in 1977) and Bacteria (or
appeared at approximately 2.3 Ga, following Eubacteria). The third domain is Eucarya (or
the rise of oxygen. These red beds indicate Eukaryota), for all eukaryotes. In this view,
higher oxygen levels because the red color animals, plants and fungi are then distant
comes from weathering of the iron in the twigs within Eucarya. Woese had generated
rocks in the presence of atmospheric oxygen. the fi rst universal tree of life (UTL). It is likely
A second rise of oxygen around 0.8–0.6 Ga is that the Archaea and Bacteria split fi rst, and
indicated by increased levels of marine sulfate. then the Eukarya split from the Bacteria, but
Oxygenated rainwater reacts with pyrite on the root of the UTL is still uncertain.
the continents and washes sulfate through Further work since 1990 has confi rmed
rivers to the oceans, so an increase in oceanic Woese’s insight, although alternative schemes
sulfate suggests an increase in oxygen. talk of two domains or six kingdoms, and
The two rises in oxygen levels, at the begin- other subdivisions. With the power of modern
ning and end of the Proterozoic, respectively, gene sequencing, it should have been rela-
mark the beginning of modern-style biogeo- tively easy to build the UTL with progres-
chemical cycles, in which oxygen and carbon sively more detail. One of the largest versions
are exchanged continuously between living of the UTL consists of 191 organisms for
organisms and the Earth’s crust. which complete genome sequences have been
established (Ciccarelli et al. 2006). However,
molecular biologists had not at fi rst contem-
The universal tree of life
plated the notion of jumping genes: simple
There used to be a quiz show on British radio organisms seem to be prone to exchanging
called Animal, vegetable or mineral? in which genes in a process called horizontal gene
a team of scientists had to identify mystery transfer. Genes can be transferred between
items. Each week, members of the public eukaryotes, but the process is commoner
would send packages of strange tubers, among prokaryotes. Horizontal gene transfer
dried internal organs and other revolting occurs in bacteria today that take up DNA
fragments for the experts to consider. The directly from their surroundings, through
division of natural objects into two living infection from a phage virus, or through
(animal, vegetable) and one non-living mating. Jumping genes make the task of the
(mineral) category reflects the common view phylogenetic sequencer difficult: parts of the
that life may be divided simply into plants genome may show linkages to one group,
(generally green, do not move) and animals while jumping genes may link the organism
(generally not green, do move). To these two to another. Once a jumping gene has been
might be added microbes (for all the micro- identified, however, it may become locked
scopic critters). into the genome of all descendants, and so
The three-kingdom view was expanded to provide evidence for the affi liation of all
four by the division of “microbes” into two organisms that possess it.
