Some time back I came across the global fossil record. This includes an account of fossil records for over six thousand species. Specific details include habitat, taxonomy, and data about when the species lived. Historical accounts span from the Hadean Precambrian to the current Holocene period based on the Geologic time scale.
The following chart lists every species on record with a timeline of its occurrences in each of the geologic time scales color coded by the habitat that species occupies.
Learning to Crawl
At least in terms of number of distinct species, the majority of life on Earth appears to be marine-based. What’s interesting is that this listing also points to a trend towards terrestrial life. Let’s test this by visualizing habitat changes over time.
The following graph displays the same information in a different way. It can be thought of as reading the rings of a tree—the beginning of paleological time is at the center with the current era represented by the outer most ring. The arc width of each color represents the percentage of species occupying that habitat at that time. Hover over a ring to focus on only on that time period and prior.
You might remember this from science class, but there is a general consensus that life began in the ocean and later moved onto land—and our data agrees. We must be on the right track.
Diversity of Life
This data also indicates that whole planet species diversity has been forever increasing.
Let’s see how this varies by habitat.
Let’s zoom back out to see how species diversity varies by kingdom over time.
And now drill down by phylum.
From this visualization we see that the most diverse phylum is chordata, which is a huge portion of what we consider the animal kingdom today. Everything from dogs to fish to birds to pogona vitticeps. Even humans are considered chordates.
There are some important limitations to be aware of in this data set. First, we only have temporal presence information, i.e. “did this species exist in this time period?”. So while we can get a sense of diversity of life by habitat, kingdom, phylum, etc, we are left to look elsewhere for population counts. Furthermore, this data does not give a sense of where these species lived—and answering that question is made more difficult by the separation of Pangea into the continents we know today.
It is also is important to recognize that this data set reveals massive gaps in our understanding of how life has evolved into what we see today. While six thousand species are included in this data set, Earth is home to an estimated 8.7 million species today. But this number doesn’t tell the whole story. Classification of organisms into species remains controversial.
Natural selection is a slow process. What we might consider minor variations can take millions of generations to evolve. This means entirely new species take even longer to emerge. It is fun to think about how our classification system would have to change to accomdate a comprehensive fossil record. Would we be coerced into inventing an entirely new system to replace the traditional kingdom/phylum/class structure? How would such a taxonomical system address these infinitesimally small variations between species?