fungi, and land animals appear; the first
dinosaurs and mammals appear; the
first flowering plants appear; the first
humans appear.4.Look at the photographs of organisms pro-
vided by your teacher. Identify the major
characteristics of each organism. Record
your observations in your data table.
5.Lay out your timeline on the floor in your
classroom. Place photographs (or draw-
ings) of the organisms you examined on
your timeline to show when they appeared
on Earth.
6.Fold the timeline at the mark representing
4.8 billion years ago. This leaves 24 seg-
ments, each representing 200 million years,
in your timeline. Now you can think of
each segment as 1 hour in a 24-hour day.
7.When you are finished, walk slowly along
your timeline. Note the sequence of events
in the history of life on Earth and the rela-
tive amount of time between each event.
PARTB:Cleanup and Disposal
- Dispose of paper scraps in the
designated waste container. - Clean up your work area and all lab
equipment. Return lab equipment to
its proper place.
Analyze and Conclude
- Analyzing Information Think of each
segment of your timeline as 1 hour in a
24-hour day as you answer each of the
following questions:
a.How long has life existed on Earth?
b.For what part of the day did only unicel-
lular life-forms exist?
c.At what time of day did the first plants
appear on Earth?
d.At what time of day did mammals
appear on Earth?
2. Summarizing InformationIdentify the
major developments in life-forms that have
occurred over the last 3.5 billion years.
3. Inferring RelationshipsHow do mass
extinctions appear to be related to the
appearance of new major groups of
organisms?
4. Justifying ConclusionsCyanobacteria
are thought to be responsible for adding
oxygen to Earth’s atmosphere. Use your
timeline to justify this conclusion.
5. CalculatingDetermine the amount of
time, as a percentage of the time that life
has existed on Earth, that humans (Homo
sapiens) have existed.
6. Further InquiryWrite a new question
about the history of life on Earth that
could be explored in another investigation.
CHAPTER 12History of Life on Earth 273
On the Job
Timelines are used to organize events in
chronological order. Do research to dis-
cover how other scientists use timelines
in their work. For more about careers,
visit go.hrw.comand type in the
keyword HX4 Careers.