History of Life on Earth

Happy New Year, planet Earth!

According to the Anno Domini designation, the year is now 2014. But the Earth has been around a lot longer than that – about 4.567 billion years. The first evidence of life dates back to around 3.8 billion years ago. Homo sapiens first appeared on the planet around two hundred thousand years – or ten thousand generations – ago.

How’s that for perspective?

Kirk Johnson, director of the National Museum of Natural History, calls this perspective “deep time.” This is the story of our planet preserved in “the DNA of living things,” Johnson explains, as well as “in the fossils we find, in the geologic structures of our planet, in the meteorites we scavenge from the ice fields in Antarctica. Those things together give us an incredible manual for thinking about the planet.”

Why is this manual useful? We are facing a century that will be an incredibly challenging one for humanity. We now live on a planet with seven billion people, which is up from 1.7 billion people just three or four generations ago. So we have more people and a greater need for resources.

Fortunately, we have the bodies of extinct plants and animals that lived for the last three-and-a-half billion years. These fossils are not only a source of energy but also a source of knowledge about how this planet works. Over its history, the Earth has seen an incredible diversity of life – maybe as many as fifty million species. Johnson says we’re learning “as much about the evolution of life on Earth by looking at what happened in the past as we are at looking at the breakthroughs in genomics and DNA of living things.” Furthermore, Johnson sees the sequencing of the human genome as the vanguard for what will eventually be “the study of the genomics of all living things.”

We have the opportunity right now, Johnson says, to choose what our future will be. Our understanding of the diversity of life on this planet, he says, will be our guide. This story is being told at a current exhibition at the National Museum of Natural History called “Genome: Unlocking Life’s Code.”

In the video, Johnson shares a unique perspective on deep time in the form of a timeline of life on this planet in just three minutes.

Transcript – Life on Earth is an amazing story. The planet forms around 4.567 billion years ago. The first rocks that appear to have any chemical evidence of life show up around 3.8 billion years ago. Then by 3.5 billion years ago we actually see evidence of these bacterial mounds. And we waited a long time before we see any life forms that are large. The first large life forms show up about 600 million years ago and they are sea floor organisms that are now extinct that looked like placemats believe it or not. After that there’s somewhere around 500 million years is an explosion of marine life, lots of diversification, the first organisms that were related to the different groups of marine animals you find today. It’s not until about 400 million years ago that the first life emerges onto land. The first little arachnid spider-like organisms, early plants that were only maybe a centimeter tall. Wait another 100 million years you get your first forest.

You get your first large bodied terrestrial animals. Things like giant millipedes and the first land living vertebrates evolving from fish. Sometime after that animals — four-legged animals finally learned how to eat plants. It took a while for the first terrestrial herbivores to appear. That happened somewhere around 300 million years ago. And then there’s a major extinction that happens at 250 million years ago. Don’t really know the cause but something to do with the perturbation of the Earth’s carbon cycle where we lose something like 90 percent of the species on the planet. When they disappear it’s like the near shave for life on Earth. But out of that grows the age of the dinosaurs. And for 150 million years we have a world that’s warm, a world that is so warm that no polar ice caps. And in that world a, a great diversity of animals that start to look familiar to you cohabit with the dinosaurs. We get the first birds. We get the first mammals. We still have these large dinosaurs and then at 66 million years ago an asteroid the size of Denver traveling 20 times the speed of a bullet crashes into the Yucatan Peninsula and causes a massive extinction of all animals that are larger than dogs on the planet.

What are The Secrets of the X chromosome? By Robin Ball

The sequence of DNA that we inherit from our parents encodes directions for making our cells and giving us specific traits. Identical twins have the same DNA sequence, so how can one twin end up with a genetic disorder while the other twin does not? Robin Ball explains how the secret lies in X chromosome inactivation.

The X chromosome is one of the two sex chromosomes in humans (the other is the Y chromosome). The sex chromosomes from one of the 23 pairs of human chromosomes in each cell. The X chromosome spans about 155 million DNA building blocks (base pairs) and represents approximately 5 percent of the total DNA in cells.

“Each person normally has one pair of sex chromosomes in each cell. Females have two X chromosomes, while males have one X and one Y chromosome. Early in embryonic development in females, one of the two X chromosomes is randomly and permanently inactivated in cells other than egg cells. This phenomenon is called X-inactivation or lyonization. X-inactivation ensures that females, like males, have one functional copy of the X chromosome in each body cell. Because X-inactivation is random, in normal females the X chromosome inherited from the mother is active in some cells, and the X chromosome inherited from the father is active in other cells.

Some genes on the X chromosome escape X-inactivation. Many of these genes are located at the ends of each arm of the X chromosome in areas known as the pseudoautosomal regions. Although many genes are unique to the X chromosome, genes in the pseudoautosomal regions are present on both sex chromosomes. As a result, men and women each have two functional copies of these genes. Many genes in the pseudoautosomal regions are essential for normal development.” https://ghr.nlm.nih.gov/chromosome/X